startx: Under Linux start X on the current VT

When we let X allocate a new VT, systemd-logind will not recognize any
processes running on this VT as belonging to a valid session (since there
was no pam session opened on that tty).

This causes problems like PolicyKit denials for these processes.

ConsoleKit under Linux has been deprecated for a few years now and is no
longer being maintained, so simply make this the default under Linux.

Note we do not pass in the vt if the user has specified an alternative server
to start, as the vtX argument is only valid for the Xorg server, likewise we
omit it if the user has specified any other server arguments.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=806491



Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>