Add upipe_rtp_prepend (prepend rtp header)

Build-dep: bitstream.
TODO:
- sequence number random init (need "urand")
- define rtp type corresponding to flowdef
- set according clock freq
Add upipe_ts_aggregate: TS packets aggregator
upipe_udp_source: move udp helper functions
upipe_udp: properly clear sin_zero

Fix sin_zero-triggered bug on some plateforms (darwin, ...)
Add upipe_udp_sink
udpmulticat: add udp_sink support
upipe_av_codec: fix compat with recent ffmpeg

Rename CodecID to AVCodecID, maintain legacy.
TODO: fix valgrind errors/memleaks with ffmpeg >= 1.2