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_api.c b/openbsc/src/osmo-bsc/osmo_bsc_api.c
index d31e6c1..7a3ef70 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_api.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_api.c
@@ -205,8 +205,8 @@
 	gsm48_tx_mm_serv_ack(conn);
 
 	LOGP(DMSC, LOGL_INFO, "Sending USSD message: '%s'\n", text);
-	gsm0480_send_ussdNotify(conn, 1, text);
-	gsm0480_send_releaseComplete(conn);
+	bsc_send_ussd_notify(conn, 1, text);
+	bsc_send_ussd_release_complete(conn);
 }
 
 /*