Allow configuring DSCP value in OsmoBTS for RTP
In OsmoMGW, it is already possible to configure the DSCP value. We also need it for OsmoBTS.
I've tried the related setsockopt() in
/src/common/rsl.c:bind_rtp() without success, it seems that it is overwritten later by libortp.
OsmoBTS uses libosmo-abis, which in turn uses libortp for the RTP connections.
I will add a osmo_rtp_socket_set_dscp() function, which calls rtp_session_set_dscp() from libortp, to libosmo-abis and then use it in OsmoBTS.