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/libbsc/Makefile.am b/openbsc/src/libbsc/Makefile.am
index 8c53817..b8e77e6 100644
--- a/openbsc/src/libbsc/Makefile.am
+++ b/openbsc/src/libbsc/Makefile.am
@@ -41,6 +41,7 @@
 	bsc_api.c \
 	bsc_msc.c bsc_vty.c \
 	gsm_04_08_utils.c \
+	gsm_04_80_utils.c \
 	bsc_init.c \
 	bts_init.c \
 	bsc_rf_ctrl.c \