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/smpp_openbsc.c b/openbsc/src/libmsc/smpp_openbsc.c
index 7e14c74..6861317 100644
--- a/openbsc/src/libmsc/smpp_openbsc.c
+++ b/openbsc/src/libmsc/smpp_openbsc.c
@@ -55,11 +55,11 @@
 
 	switch (npi) {
 	case NPI_Land_Mobile_E212:
-		subscr = subscr_get_by_imsi(net, addr);
+		subscr = subscr_get_by_imsi(net->subscr_group, addr);
 		break;
 	case NPI_ISDN_E163_E164:
 	case NPI_Private:
-		subscr = subscr_get_by_extension(net, addr);
+		subscr = subscr_get_by_extension(net->subscr_group, addr);
 		break;
 	default:
 		LOGP(DSMPP, LOGL_NOTICE, "Unsupported NPI: %u\n", npi);