mgcp/rtp: Add RTP header patch mode configuration

This adds datastructures and a VTY frontend to configure the
different type of RTP header patching: SSRC and timestamp.

Note that timestamp patching is not yet implemented.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/libmgcp/mgcp_network.c b/openbsc/src/libmgcp/mgcp_network.c
index 53f1a20..367cebd 100644
--- a/openbsc/src/libmgcp/mgcp_network.c
+++ b/openbsc/src/libmgcp/mgcp_network.c
@@ -264,7 +264,7 @@
 		state->seq_offset = (state->out_stream.last_seq + 1) - seq;
 		state->timestamp_offset =
 			(state->out_stream.last_timestamp + tsdelta) - timestamp;
-		state->patch = endp->allow_patch;
+		state->patch = rtp_end->force_constant_ssrc;
 		LOGP(DMGCP, LOGL_NOTICE,
 			"The SSRC changed on 0x%x SSRC: %u offset: %d tsdelta: %d "
 			"from %s:%d in %d\n",