logind: require VTs on seat0 and forbid elsewhere

logind: require VTs on seat0 and forbid elsewhere

Sessions on seat0 must pass us a vtnr, otherwise, you shouldn't try
attaching it to seat0. For seats without VTs, we do the exact opposite: we
forbid VTs.

There can be odd situations if the session-files contain invalid
combinations. However, we try to keep sessions alive and restore state as
good as possible.