bsc: Add a ctrl command to send a ussdNotify for a call

Send a non-call related SS message for an active call indentified
by the CIC of that call. As an ugly hack the order of the SS
release and the invocation are changed. That was necessary for the
E71 on a TCH. The time between notify and release was just too short.
The right would be to wait for the returnResultLast but this would
involve keeping more local state. Let's see how far we get here. It
might be necessary to change the order in the other call sites as
well.
diff --git a/openbsc/src/libbsc/bsc_api.c b/openbsc/src/libbsc/bsc_api.c
index 04a4619..5ce0bc4 100644
--- a/openbsc/src/libbsc/bsc_api.c
+++ b/openbsc/src/libbsc/bsc_api.c
@@ -857,3 +857,8 @@
 {
 	osmo_signal_register_handler(SS_LCHAN, bsc_handle_lchan_signal, NULL);
 }
+
+struct llist_head *bsc_api_sub_connections(struct gsm_network *net)
+{
+	return &sub_connections;
+}