It's called conversion-cache for now, as it will also be used for
ImageMagick/dcraw-sourced images in the future. This should also help
avoid confusion between HTTP/ImageMagick/dcraw cache (files on disk),
Imlib2 cache (images in memory), and thumbnail cache.

Closes #506

This ensures that X11 layout or other changes do not affect the .fehbg
contents, which are meant to only be updated when a --bg-option is used
manually.

Closes #482

Small fixes in man page.

This changes the behaviour of "feh --start-at FILENAME" as follows:

Previously, this would cause feh to load all files in the current
working directory, since no files or filelists were specified on the command
line.

Now this is a special case: feh will extract the directory component from
FILENAME, load all files in that directory, and display FILENAME.
This way, it's possible to use "feh --start-at ~%f" in XDG Desktop files
and browse files in feh using the prev/next keys

Closes #372
Closes #420

This more closely resembles the expected behaviour of --start-at, but
may lead to mismatches if several files in the filelist have the same
basename.

Closes #206

Related to #372 and #420

This is in line with other software. save_filelist now defaults to "L".

closes #431

This restores pre-2.21 ~/.fehbg behaviour. Replicating the entire commandline
in ~/.fehbg turned out to do more harm than good.

Closes #398

  man -Tlint feh

  STYLE: no blank before trailing delimiter: Em Note:
  WARNING: skipping paragraph macro: Pp after Sh

  no blank before trailing delimiter
    (mdoc) The last argument of a macro that supports trailing
    delimiter arguments is longer than one byte and ends with a
    trailing delimiter. Consider inserting a blank such that the
    delimiter becomes a separate argument, thus moving it out of
    the scope of the macro.

  skipping paragraph macro
    In mdoc(7) documents, this happens

      - at the beginning and end of sections and subsections
      - right before non-compact lists and displays
      - at the end of items in non-column, non-compact lists
      - and for multiple consecutive paragraph macros.

    In man(7) documents, it happens

      - for empty P, PP, and LP macros
      - for IP macros having neither head nor body arguments
      - for br or sp right after SH or SS

  https://man.openbsd.org/mandoc.1

  man: feh.1:1483:9: STYLE: no blank before trailing delimiter: Em Note:
  man: feh.1:1897:2: WARNING: skipping paragraph macro: Pp after Sh

There are two warnings here.

  1. This is fixed by adding a zero-width space (\&) so that the
     trailing delimiter character (:) is no longer at the end.
     This also adds this to a few other similar examples. The man page
     should render the same after this change.

  2. Removed an extra .Pp macro as suggested by the warning, this does
     not change how the man page renders.

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