GSUP/SMS: introduce READY-FOR-SM message

According to 3GPP TS 29.002, section 12.4, MAP-READY-FOR-SM is
used between the MSC and VLR as well as between the VLR and the
HLR to indicate that a subscriber has memory available for SMS.

This change replicates this service in GSUP as READY_FOR_SM_*.
The only mandatory IE for this service (excluding Invoke ID) is
'Alert Reason' that is replicated by OSMO_GSUP_SM_ALERT_RSN_IE.

Change-Id: Ic37f3b2114b8095cfce22977e67133b9103942e3
Related Change-Id: (docs) I549b6c8840a1e86caac09e77fb8bc5042d939e62
Related Change-Id: (TTCN) If2256607527ecfcb10285583332fb8b0515d7c78
Related: OS#3587
diff --git a/tests/gsup/gsup_test.ok b/tests/gsup/gsup_test.ok
index 7a9455d..36e35c8 100644
--- a/tests/gsup/gsup_test.ok
+++ b/tests/gsup/gsup_test.ok
@@ -41,4 +41,6 @@
           MO-/MT-ForwardSM Response OK
   Testing MO-/MT-ForwardSM Error
           MO-/MT-ForwardSM Error OK
+  Testing ReadyForSM (MSC -> SMSC) Indication
+          ReadyForSM (MSC -> SMSC) Indication OK
 Done.