We have added support for sending SDP via MNCC a long time ago, but so
far the SDP section remained empty. Now, implement actually forwarding
SDP codec information between SIP and MNCC.
The aim is to let the MSC know about all codec choices the remote SIP
call leg has to offer, so that finding a codec match between local and
remote call leg becomes possible.
Store any SDP info contained in incoming SIP and MNCC messages, and send
the stored SDP to the other call leg in all outgoing SIP and MNCC
messages.
In sdp_create_file(), we used to compose fixed SDP -- instead, take the
other call leg's SDP as-is, only make sure to modify the mode (e.g.
"a=sendrecv") to reflect the current call hold state.
The RTP address and codec info in the MNCC structures is now essentially
a redundant / possibly less accurate copy of the SDP info, but leave all
of that as-is, for backwards compat.
There is codec checking that may reject unexpected codecs. The
overall/future aim is to leave all codec checking up to the MSC, but so
far just leave current behaviour unchanged, until we notice problems.
Related: SYS#5066
Related: osmo-ttcn3-hacks Ib2ae8449e673f5027f01d428d3718c006f76d93e
Change-Id: I3df5d06f38ee2d122706a9ebffde7db4f2bd6bae
d50637eec6