Adds Alloc(), the ephemeral loopback UDP port grabber the subsystem
uses to pick the RTP port it will hand to FFmpeg and then re-bind with
core/webrtc.NewSourceOn. Covered by a 100x rebind test.
Adds BuildArgs(), which emits the -f rtp output fragments (video on
the passed port, audio on port+1) with copy codecs by default and an
H.264 baseline / libopus re-encode leg when ForceTranscode is set.
Covered by three unit tests.
