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

Closes #545

The --version-sort option requires strverscmp(), but this is a glibc
extension that does not exist on OpenBSD and other systems. To enable
--version-sort on those systems, provide an internal implementation of
strverscmp(). The implementation is from the musl C library and is
MIT-licensed.

The build process remains the same: the verscmp flag should be set to 1
only if strverscmp() is available in libc. If verscmp is 0, then the
internal implementation is used.

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