mgcp: Begin handling of the RQNT message as needed for DTMF

Introduce a callback for the request and forward the signalrequest
to the callback. This is not a full implementation of MGCP RQNT.
diff --git a/openbsc/tests/mgcp/mgcp_test.ok b/openbsc/tests/mgcp/mgcp_test.ok
index e61c0bc..1143a04 100644
--- a/openbsc/tests/mgcp/mgcp_test.ok
+++ b/openbsc/tests/mgcp/mgcp_test.ok
@@ -7,6 +7,8 @@
 Testing SHORT2
 Testing SHORT3
 Testing SHORT4
+Testing RQNT1
+Testing RQNT2
 Testing DLCX
 Testing packet loss calculation.
 Done