use /run instead of /var/run as ROOT

ToT now uses /run in anticipation of a udev uprev, with /var/run
being a symlink for compatibility. fixabslinks assumes no symlink
in the CHROOT variable, so /run should be used directly when
passing paths to fixabslinks.
better setting of ROOT in {un,}mount-chroot
Merge pull request #907 from smibarber/rundir

Use /run instead of /var/run as ROOT