factor out gen of USSD notify and release complete to libosmocore

Both libmsc and libbsc will need distinct gsm0480_send_ussdNotify() and
gsm0480_send_releaseComplete() functions, since there will be distinct
subscriber connection structs.

Rename to msc_send_ussd_notify() and msc_send_ussd_release_complete(), and add
the same in libbsc with bsc_ prefix in new file gsm_04_80_utils.c.

In preparation of this patch, the message generation part of these functions
has been added to libosmocore as gsm0480_create_ussd_notify() and
gsm0480_create_ussd_release_complete(). Use these.

Adjust all libmsc and libbsc callers according to use the msc_* or bsc_*
implementation, respectively.

Change-Id: I33a84e3c28576ced91d2ea24103123431f551173
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_grace.c b/openbsc/src/osmo-bsc/osmo_bsc_grace.c
index e6194ab..6409a3a 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_grace.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_grace.c
@@ -112,8 +112,8 @@
 	if (lchan->state != LCHAN_S_ACTIVE)
 		return -1;
 
-	gsm0480_send_ussdNotify(conn, 0, text);
-	gsm0480_send_releaseComplete(conn);
+	bsc_send_ussd_notify(conn, 0, text);
+	bsc_send_ussd_release_complete(conn);
 
 	return 0;
 }