refactor: use msgb to receive, pass and send RTP packets

Instead of numerous arguments (buf, len and context data), use a msgb, like
most other osmo programs do, with a msb->cb pointing at a context data struct.

This opens the future for adding/stripping IuUP header data from the msgb
easily.

(Checked to pass current ttcn3-mgw-tests.)

Change-Id: I3af40b63bc49f8636d4e7ea2f8f83bb67f6619ee
7 files changed