slideshow: When removing last image, stay on the last image

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.

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

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)