x11/xdm: minor fixes

When testing, the rc script worked fine on my machine with no extra ttys,
but when on a default installation with several vtys it conflicts with ttyv1.
This script defaults to activating on ttyv8 as this is the one xdm usually
is on by default and is the usual Ctrl+Alt+F9 for X.

Add missed PORTREVISION (thanks mat)