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/silent_call.c b/openbsc/src/libmsc/silent_call.c
index 4462dfc..010c2b4 100644
--- a/openbsc/src/libmsc/silent_call.c
+++ b/openbsc/src/libmsc/silent_call.c
@@ -120,7 +120,7 @@
 {
 	int rc;
 
-	rc = paging_request(subscr->net, subscr, type,
+	rc = paging_request(subscr->group->net, subscr, type,
 			    paging_cb_silent, data);
 	return rc;
 }