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)

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.