seat: Open/close tty on activation/deactivation

The VT and KD ioctl's are picky about the tty fd used. In order to
satisfy these, and to improve state cleanup, we now only and store the
current tty when opening a client, and use this fd to perform teardown
later. The presence of the fd is also used to signal that teardown is
needed.

include/seat.h

@@ -32,6 +32,7 @@ struct seat {
 	bool vt_bound;
 	bool vt_pending_ack;
 	int next_vt;
+	int curttyfd;
 };
 
 struct seat *seat_create(const char *name, bool vt_bound);