mgcp: Add a call tap feature to forward audio to another forward port

For debugging it is useful to forward (tee) UDP packets to another
system and use gstreamer to inspect the rtp stream. This is untested
code and might contain bugs.... and of course only tap your own calls.
diff --git a/openbsc/src/mgcp/mgcp_protocol.c b/openbsc/src/mgcp/mgcp_protocol.c
index 036ddf0..9916b40 100644
--- a/openbsc/src/mgcp/mgcp_protocol.c
+++ b/openbsc/src/mgcp/mgcp_protocol.c
@@ -826,4 +826,6 @@
 
 	endp->conn_mode = endp->orig_mode = MGCP_CONN_NONE;
 	endp->allow_patch = 0;
+
+	memset(&endp->taps, 0, sizeof(endp->taps));
 }