imlib.c: Use wget --no-clobber

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 23421a86)

Conflicts:

	ChangeLog