gsm0480: add gsm0480_create_ussd_notify() and *_release_complete()

Add two functions to create USSD messages. Moves and generalizes code from
openbsc. Pending: use the new functions in openbsc.

It looks like _release_complete() should also set trans_id and direction flag;
but since this is moving code from openbsc that is apparently working, just
place a fixme comment and don't change the functionality.

Change-Id: Ia80e32c7105359915bfad3cc5621a1c09caf20f0
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index f0baa21..3e7333c 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -63,6 +63,8 @@
 gsm0480_create_notifySS;
 gsm0480_create_unstructuredSS_Notify;
 gsm0480_create_ussd_resp;
+gsm0480_create_ussd_notify;
+gsm0480_create_ussd_release_complete;
 gsm0480_decode_ussd_request;
 gsm0480_decode_ss_request;
 gsm0480_wrap_facility;