Two commits ago I tried to only strip the consolekit prefix
off the seat id, if there was a consolekit prefix.  In the
process I ended up not initializing the seat_id variable
at all in the non-consolekit case.

This commit fixes that.

Now that we can use logind for seat management, there' may not be
/org/freedesktop/ConsoleKit in the seat identifier.

This commit makes sure the GDM_SEAT_ID environment variable doesn't
show gunk in the case the system is configured to use logind.

Related downstream report:

http://bugzilla.redhat.com/810451

Don't show locked (disabled) users in the user list.
Those are either "system" users with a shell or users disabled in the control-center.

See: https://bugzilla.gnome.org/show_bug.cgi?id=673784

Make session unlock work with logind.
This was not working due to a typo in the bus name of logind.

https://bugzilla.gnome.org/show_bug.cgi?id=673620

commit b4367642 tried to prevent
autologin on auxilliary seats, but the conditional was inverted,
so it ended up preventing autologin on the primary seat.

https://bugzilla.gnome.org/show_bug.cgi?id=672773

commit fb6a3cf2 fixed
user-switching so that we always migrate to a pre-existing
session if it's runninng.

Unfortunately, we didn't filter out tty sessions.

This commit ensures GDM only jumps to graphical sessions.

https://bugzilla.gnome.org/show_bug.cgi?id=672630

ck-x11-get-display-device is a soft dependency.  If it's installed
then pam_ck_connector will function properly, otherwise it won't.

Since it's a soft dependencies, we should do it even if building
--disable-console-kit .  This allows distros to drop the ConsoleKit
buildrequires and still keep support for consolekit via
pam_ck_connector.