ASCI: Add new mode for voice group/broadcast call

The new mode "confecho" is similar to "sendrecv", except that it also
echoes back RTP towards the sender. This is required for voice group or
broadcast calls. Talker and listeners use the same timeslot, so that
audio must be echoed from the talker to the listeners.

It is different from "loopback", because a loopback only echoes back RTP
towards the sender, but does not forward audio through the endpoint to
the other connections. Also it does not forward RTP from senders of
other connections.

The current implementation of MGW does not support transcoding and
mixing. This means that a sending connection must not send RTP that has
been received by multiple receiving connections. The application that
uses the MGW has to set the connection modes, so that only one
connection receives RTP in case of a conference.

Change-Id: I0639c663e119d85bef1010c7aa45e2f133a9daf0
Related: OS#4853
diff --git a/include/osmocom/mgcp/mgcp_common.h b/include/osmocom/mgcp/mgcp_common.h
index 07d8d37..7de45f9 100644
--- a/include/osmocom/mgcp/mgcp_common.h
+++ b/include/osmocom/mgcp/mgcp_common.h
@@ -47,6 +47,7 @@
 	MGCP_CONN_SEND_ONLY = 2,
 	MGCP_CONN_RECV_SEND = MGCP_CONN_RECV_ONLY | MGCP_CONN_SEND_ONLY,
 	MGCP_CONN_LOOPBACK  = 4 | MGCP_CONN_RECV_SEND,
+	MGCP_CONN_CONFECHO  = 8 | MGCP_CONN_RECV_SEND,
 };
 
 #define MGCP_X_OSMO_IGN_HEADER "X-Osmo-IGN:"