This fixes the following mandoc warnings.

  man -Tlint feh

  WARNING: skipping empty macro: No

  skipping empty macro
    (mdoc) The indicated macro has no arguments and hence no effect.

  man: /tmp/man1/feh.1:16:14: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:16:34: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:179:2: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:387:48: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:785:26: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:785:38: WARNING: skipping empty macro: No
  man: /tmp/man1/feh.1:823:26: WARNING: skipping empty macro: No

This fixes many warnings shown by mandoc lint feature and should also respect
the 80 col limit.

  man -Tlint feh

  WARNING: new sentence, new line

  new sentence, new line
    (mdoc) A new sentence starts in the middle of a text line. Start it on a new
    input line to help formatters produce correct spacing.

man: /usr/man/man1/feh.1.gz:35:32: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:78:33: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:90:50: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:95:53: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:97:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:108:17: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:112:36: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:115:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:122:20: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:129:21: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:130:9: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:135:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:146:64: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:148:43: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:149:58: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:167:2: WARNING: skipping empty macro: No
man: /usr/man/man1/feh.1.gz:175:21: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:177:30: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:178:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:186:42: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:189:15: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:199:49: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:213:6: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:227:47: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:231:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:242:11: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:274:66: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:285:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:297:9: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:311:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:340:21: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:351:14: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:360:69: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:365:44: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:372:35: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:395:21: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:396:16: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:397:31: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:406:65: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:407:53: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:430:11: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:440:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:442:11: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:447:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:455:10: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:458:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:491:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:492:44: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:493:42: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:498:26: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:511:59: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:512:55: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:530:18: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:531:46: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:532:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:533:41: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:538:50: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:545:13: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:562:11: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:569:41: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:576:26: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:584:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:607:49: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:616:61: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:617:76: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:623:61: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:635:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:638:27: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:640:37: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:650:17: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:666:37: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:677:42: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:682:20: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:683:8: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:688:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:703:51: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:714:37: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:728:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:729:44: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:738:61: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:775:34: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:778:42: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:787:36: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:822:19: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:823:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:852:46: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:868:26: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:881:10: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:889:10: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:902:43: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:920:65: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:934:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:954:69: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:965:37: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:973:65: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:987:17: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1027:29: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1056:17: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1143:28: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1164:38: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1171:49: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1240:54: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1271:58: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1289:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1291:57: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1292:59: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1293:53: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1318:32: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1329:35: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1335:13: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1341:18: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1349:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1358:24: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1391:44: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1398:25: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1399:8: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1405:67: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1408:23: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1569:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1680:21: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1681:70: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1683:22: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1708:55: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1723:36: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1742:54: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1743:45: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1771:67: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1816:35: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1822:14: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1845:47: WARNING: new sentence, new line
man: /usr/man/man1/feh.1.gz:1902:56: WARNING: new sentence, new line

Closes #386

Closes #272

Closes #218

FWIW: This behaviour was already present before
394517d1

Closes #326

closes #302

closes #270

Many image collections are organized by directory, so it is nice to have
jump-to-adjacent-directory navigation.

e.g. Given the following file hierarchy:

    .
    ├── A
    │   ├── 1.jpg
    │   ├── 2.jpg
    │   └── C
    │       ├── 1.jpg
    │       ├── 2.jpg
    │       └── 3.jpg
    └── B
        ├── 1.jpg
        ├── 2.jpg
        └── 3.jpg

`feh --recursive` creates the following filelist:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

If we press [next_dir], we move the current_file pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg <-- current_file
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [next_dir] again moves the pointer to:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

[next_dir] now moves the pointer back to the top of the list:

    A/1.jpg <---- current_file
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg
    B/2.jpg
    B/3.jpg

Pressing [prev_dir] from here moves backwards to the first image of the
previous directory:

    A/1.jpg
    A/2.jpg
    A/C/1.jpg
    A/C/2.jpg
    A/C/3.jpg
    B/1.jpg <---- current_file
    B/2.jpg
    B/3.jpg

When starting from an position that is not the first image of a
directory, [prev_dir] moves the pointer to the first image of the
current directory.

These actions combine well with `--sort dirname` since all regular files
in a directory will be sorted before any subdirectories, avoiding a
filelist like the following:

    A/1.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg
    A/4.jpg

With `--sort dirname` that filelist becomes:

    A/1.jpg
    A/4.jpg
    A/SUBDIR/2.jpg
    A/SUBDIR/3.jpg

Sort filelist by dirname, then by name. This results in file entries
sorting before subdirectory entries.

Useful in conjunction with upcoming prev_dir and next_dir navigation
actions.