Bug fix - rxvt-unicode works only for one user.

This problem was solved by forcing rxvt-unicode to put its socket in a specific
path, so /usr/bin/urxvt can check whether urxvtd is already running by checking
whether it exists, instead of using pidof.
Added a file dropped from the previous commit.
Bug fix - allow non-root users to Debian's X server.
By default, allow all users to mount and unmount.

This reduces security, as it allows easy privilege escalation (using the
replacement of executables executed by root). However, it allows easy and clean
integration of part-icon-handler, without sudo.