030-wm.sh: switch to sudo to fix controlling tty issue with recent X.org

X.org doesn't run as root anymore (unless using
xserver-xorg-legacy workaround). When invoking X.org via 'su -c
...' no controlling tty can be acquired, because of a change in
upstream's shadow release 4.1.5 (dating back to 2011-06-05).
Related to
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802769
We need read and write permissions on the TTY, so adjust the TTY
we're running on accordingly.