gms_transactions data model reorganization

This changeset factors out gsm_transaction as something independent
of call control in preparation to re-use the code from SMS.  A
transaction is uniquely identified by either its callref, or by
a tuple of (transaction_id, protocol, subscriber).
diff --git a/openbsc/src/gsm_subscriber.c b/openbsc/src/gsm_subscriber.c
index 1a5ea2f..60dec43 100644
--- a/openbsc/src/gsm_subscriber.c
+++ b/openbsc/src/gsm_subscriber.c
@@ -171,6 +171,7 @@
 	/* FIXME: Migrate pending requests from one BSC to another */
 	switch (reason) {
 	case GSM_SUBSCRIBER_UPDATE_ATTACHED:
+		s->net = bts->network;
 		/* Indicate "attached to LAC" */
 		s->lac = bts->location_area_code;
 		break;