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/osmo-bsc/osmo_bsc_bssap.c b/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
index dda3157..a801e0e 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_bssap.c
@@ -167,7 +167,7 @@
 		LOGP(DMSC, LOGL_ERROR, "eMLPP is not handled\n");
 	}
 
-	subscr = subscr_get_or_create(msc->network, mi_string);
+	subscr = subscr_get_or_create(msc->network->subscr_group, mi_string);
 	if (!subscr) {
 		LOGP(DMSC, LOGL_ERROR, "Failed to allocate a subscriber for %s\n", mi_string);
 		return -1;