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)

 * general
 * commandline option overrides same theme option
 * multiline theme definitions