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>

The 'fehbg' variable can be allocated whether or not the user wishes to
generated a fehbg file. Ensure the variable is free'ed before we return.

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

This way, the current image settings are used when enabling keep_zoom_vp.
It used to reset the current image to default, which probably wasn't very
intuitive.

manually changed in collage, imlib, filelist, thumbnail, slideshow,
index
feh_display_status also changed

this introduces a bug in feh_wm_set_bg, which will be fixed in the next
commit (only applies when run without HOME)

Note that the i == r check was never reached in the original code but may be
in the new variant. Since a shuffled list may well have one or two elements
retain their absolute position once in a while, this should not be a problem.

tests with 2- and 3-element lists show an even distribution amongst all
elements.

Not all functions were examined yet. Also TODO:
* check strdup / malloc calls (some are apparently unchecked in giblib),
  maybe use estrdup / emalloc for those
* Check includes and update dependency list if needed

This means it will be treated like an HTTP image (no write operations,
copying to /tmp etc), but it's still better than no file:// support.

Also, this is why it's not documented yet.