This prevents a (highly unlikely) case of an attacker knowing feh's PID and
the user's URL rewriting user files by means of a TOCTTOU attack.

It is still possible to _create_ arbitrary files via dangling symlinks. That
will be fixed once I switch from wget to libcurl.

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.

Ugly workaround, gotta make it nicer some time