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.

(cherry picked from commit 3bd5012d)

Fixes the annoying 100% zoom and then continuing where it was.

This way, zooming, panning etc. gets sped up quite a bit.

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.