This fixes a really weird bug where continued theme definitions containing
just one option/value pair caused feh to mis-parse parts of the following
line.

Apparently, subsequent realloc calls mixed with strdups in another function
caused parts of the list (argv) content to be overwritten by the content of
the list pointer itself. I wasn't able to find out the exact causes /
conditions.

In feh_parse_option_array, when a hold-action is set from a theme it would
behave differently than one set from commandline.

Theme: ;;foo would become foo (incorrect)
cmdline: ;;foo would become ;foo (correct)

I doubt anybody would ever have noticed it, since ;foo is a shell syntax error
anyways, but it's definitely better to be on the safe side.

When executing an action starting with a semicolon, feh will
not skip to the next slide. The semicolon is not passed on to
the executing shell, of course. This is kinda ugly, but at least
it makes action handling somewhat more flexible.

Option --action-hold-slide. If specified, when in slideshow mode images will not automatically change after running user action. Otherwise, slideshow images will cycle after action as usual.