Merge in several recent RTPproxy related features from the iptel's SER.
o New rtpproxy_offer() and rtpproxy_answers() functions whose purpose is
similar to force_rtp_proxy's 'S' flag (to allow SDP-less INVITEs), but
providing cleaner API.
o New family of functions rtpproxy_stream2uac(), rtpproxy_stream2uas(),
rtpproxy_stop_stream2uac() and rtpproxy_stop_stream2uas() allowing to
play pre-recorded announcement/MOH to one or both parties in response
to various events. This feature requires latest 1.2 version snapshor
of RTPproxy available at http://www.rtpproxy.org
o Inclease list of recognizable and proxable streams with rtp/avpf,
rtp/savp and udp/bfcp.
o Move some of the functions out of the overly long nathelper.c.