[paging] Use one of the two reserved LAC to page every BTS

For the on-waves.com MSC case we want to page every BTS reached
of the network. Our gsm_subscriber entry does not have a LAC
entry set and defaults to zero. Use the reserved 0x0000 to
indicate that we want to use every bts in the network.

This will influence the paging code to start and stop paging.

openbsc/src/gsm_data.c

@@ -274,7 +274,7 @@ struct gsm_bts *gsm_bts_by_lac(struct gsm_network *net, unsigned int lac,
 			continue;
 		}
 
-		if (bts->location_area_code == lac)
+		if (lac == 0 || bts->location_area_code == lac)
 			return bts;
 	}
 	return NULL;