slave_run: do not run the screen as root

Signed-off-by: Andreas Beckmann <anbe@debian.org>
use screen -ls to check for running screens

Signed-off-by: Andreas Beckmann <anbe@debian.org>
slave_join: use 'script /dev/null' to get a writable tty

avoid

user$ sudo su - piupartss
piupartss$ screen -x
Cannot open your terminal '/dev/pts/42' - please check.

Signed-off-by: Andreas Beckmann <anbe@debian.org>