Closes #584

Closes #584

This is kinda sloppy coding (feh --filelist and --bg-* will fail when running
on a system with path lengths >= 4096 and PATH_MAX not defined), but that's
sufficiently improbable to be okay. (plus, I ain't getting paid for this, so
if you want to improve it feel free to do so)

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.

Sort filelist by modification time, newer files first. Useful for
perusing a directory of images by most recently added or changed.

  than the filesystem's (possibly random) order.