Improve page command handling in the bsc_hack

Add a callback to the gsm_network. When updating the location and
assigning a new tmsi callback into the bsc_hack.c and have a queue
of mobile stations to page, allocate a channel for and ultimately
dial.
diff --git a/include/openbsc/gsm_data.h b/include/openbsc/gsm_data.h
index 9c0ac55..8f21524 100644
--- a/include/openbsc/gsm_data.h
+++ b/include/openbsc/gsm_data.h
@@ -144,6 +144,8 @@
 	struct gsm_bts	bts[GSM_MAX_BTS+1];
 	struct gsm_ms	*ms;
 	struct gsm_subscriber *subscriber;
+
+	void (*update_request_accepted)(struct gsm_bts *, u_int32_t);
 };
 
 struct gsm_network *gsm_network_init(unsigned int num_bts, u_int16_t country_code,