mscsplit: move subscriber conns list into struct gsm_network

Replace the global sub_connections llist with gsm_network.subscr_conns.
Initialize and apply where applicable.

Remove bsc_api_sub_connections(), callers now access gsm_network->subscr_conns
directly.

This allows using the subscr_conns from libmsc without having to link libbsc.

Change-Id: Ice2a7ca04910bcfaaff22539abe68a6349e8631c
diff --git a/openbsc/src/libbsc/net_init.c b/openbsc/src/libbsc/net_init.c
index 65419ae..e01ba80 100644
--- a/openbsc/src/libbsc/net_init.c
+++ b/openbsc/src/libbsc/net_init.c
@@ -79,6 +79,7 @@
 	INIT_LLIST_HEAD(&net->trans_list);
 	INIT_LLIST_HEAD(&net->upqueue);
 	INIT_LLIST_HEAD(&net->bts_list);
+	INIT_LLIST_HEAD(&net->subscr_conns);
 
 	/* init statistics */
 	net->bsc_ctrs = rate_ctr_group_alloc(net, &bsc_ctrg_desc, 0);