port and multicast conventions for medianet #130

Port conventions

29998/udp H264 video via GStreamer udpsource/sink
29999/udp MJPEG video via GStreamer udpsource/sink
30000/udp zita-njbridge n-channel, 24bit audio @ 48000 kHz

Multicast group conventions

239.192.18.0/23 admin scope for medianet use
239.192.18.1/26 ad-hoc use
239.192.18.65/26 standalone audio streams
239.192.18.127/32 party-mode
239.192.18.128/32 all receivers
239.192.18.129/26 video streams
239.192.18.173/26 video-related audio streams