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/include/openbsc/gsm_data.h b/openbsc/include/openbsc/gsm_data.h
index 9317717..07db02f 100644
--- a/openbsc/include/openbsc/gsm_data.h
+++ b/openbsc/include/openbsc/gsm_data.h
@@ -345,6 +345,9 @@
 	/* Allow or disallow TCH/F on dynamic TCH/F_TCH/H_PDCH; OS#1778 */
 	bool dyn_ts_allow_tch_f;
 	/* TODO: vty for this; related: OS#1781 */
+
+	/* all active subscriber connections. */
+	struct llist_head subscr_conns;
 };
 
 struct osmo_esme;