kmscon: run only on VT-less seats in listen-mode

kmscon: run only on VT-less seats in listen-mode

If we run in listen mode, we are supposed to take over a seat. If a seat
supports VTs (like kernel VTs or kmscon cdev fake VTs) we assume that
there is some manager for these VTs (the one who created them). Therefore,
there is no need to run kmscon in listen mode on these seats. Instead, you
should run kmscon in default mode on these seats.

We enforce this limitation because if the VT-master on those seats dies
and causes a HUP, they have no way to notify us when they startup again.
Therefore, this kind of setup is broken. Furthermore, no-one would every
want such setups.

Instead use the startup mechanism of the VT/seat-manager to start kmscon
in default mode on those seats.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>