msc: Add and use gsm_subscriber_group

Currently every subcriber object directly refers to the gsm_network
which contains a flag shared by every related subscriber
(keep_subscr). This adds a dependency on gsm_network even if only the
function defined in gsm_subscriber_base.c are used.

This patch adds a new struct gsm_subscriber_group which contains the
keep_subscr flag and a back reference to the network object. The
latter is not dereferenced in gsm_subscriber_base.c, so it can safely
be set to NULL when only that part of the gsm_subscriber API is being
used. It also changes that API to use gsm_subscriber_group instead of
gsm_network parameters.

Since there are some places where a pointer to the gsm_network is
needed but where only a gsm_subscriber is available, a 'net' back
pointer is added to the group struct, too. Nevertheless subscr group
and network could be separated completely, but this is not the topic
of this commit.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/libmsc/db.c b/openbsc/src/libmsc/db.c
index 656c661..bdecbb4 100644
--- a/openbsc/src/libmsc/db.c
+++ b/openbsc/src/libmsc/db.c
@@ -1470,7 +1470,7 @@
 		strncpy(sms->dst.addr, daddr, sizeof(sms->dst.addr));
 		sms->dst.addr[sizeof(sms->dst.addr)-1] = '\0';
 	}
-	sms->receiver = subscr_get_by_extension(net, sms->dst.addr);
+	sms->receiver = subscr_get_by_extension(net->subscr_group, sms->dst.addr);
 
 	sms->src.npi = dbi_result_get_ulonglong(result, "src_npi");
 	sms->src.ton = dbi_result_get_ulonglong(result, "src_ton");
@@ -1597,7 +1597,7 @@
 		return NULL;
 	}
 
-	sms = sms_from_result(subscr->net, result);
+	sms = sms_from_result(subscr->group->net, result);
 
 	dbi_result_free(result);