sig_exit may be changed by a signal handler, so its value should always be
read from RAM.

See also #435

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

Merge branch 'output-directory-for-image-saving' of https://github.com/Ferada/feh into Ferada-output-directory-for-image-saving

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