Fix some warnings from `gcc`.

Closes #595

.fehbg now stores the full path to the feh binary when it was run using a relative path (i.e., not from $PATH)

Closes #584

feh is C code, _XFUNCPROTO(BEGIN|END) are only needed for C++ applications.

Closes #584

feh is C code, _XFUNCPROTO(BEGIN|END) are only needed for C++ applications.

Merge branch 'add-windowid-option' of https://github.com/codehearts/feh into codehearts-add-windowid-option

Closes #583

Fixes https://github.com/derf/feh/issues/580

Note that Imlib2 does not support HEIC/HEIF images out of the box. However, the
imlib2-heic loader (https://github.com/vi/imlib2-heic) does.

Closes #579

Closes #577

This fixes two memory bugs that only manifest with exif=1 and long-running
slideshows.

* when feh loads an image, it writes exif data to file->ed. Previously, this
  data was never free'd, causing a memory leak on subsequent loads of the same
  file.
* As file->ed is never free'd, the accumulated EXIF data consumes a significant
  amount of memory over time. with slideshow-delay = 10 and two days of
  runtime, feh may exceed 1 GB of memory usage. If the slideshow is so large
  that feh does not encounter the same image twice in this time, this is not
  detected as a memory leak, as each EXIF data chunk is referenced from the
  filelist.

See <https://github.com/derf/feh/issues/553> for details.

Closes #553

If --edit is set, it is indeed "Edit in Place". Otherwise, it changes the
view, but not the image on the disk.

man: Silence mandoc warning.

This works around a regression in Imlib2, which makes (un)loadable file
detection quite slow when handling e.g. large video files. See
<https://phab.enlightenment.org/T8739> and
<https://github.com/derf/feh/issues/505> for details.

Closes #505

feh.1:588:33: WARNING: new sentence, new line