config: provide example configuration for multi-seat setups

config: provide example configuration for multi-seat setups

Seats other than seat0 need custom configuration. Provide that with a
default configuration file so we can share it across distros.

This file intentionally does not end in .conf so it won't get picked up by
the server after a normal installation. gdm, or whatever starts up the
servers will have to explicitly specifiy this config file.

This file replaces the one currently written by systemd's multi-seat-x
binary:
http://cgit.freedesktop.org/systemd/systemd/tree/src/login/multi-seat-x.c

CC: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Dan Nicholson <dbn.lists@gmail.com>