The following adds the ability, when using `feh` as a background setter,
to use a specific Xinerama screen to render a provided image. Along with
the provided '--bg-*' option, the '--xinerama-index' argument will be
used to identify the index of the detected Xinerama screen to use.

Signed-off-by: James Knight <james.d.knight@live.com>

While at it: Add some notes about .fehbg in general and the fact that it is
an executable shell script now.

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

Patch by sdaau on IRC. Thanks!