Rework ORTA/ORTV. Fixes #66, #64.

ORTA/ORTV can now be set to a number smaller than the number of MSTs,
leading to a=sendonly settings.
Also allowed VAD to work in CreateAnswer. (#64)
Because we may end up recycling a m= line, I don't think we can say "most recent m= sections"; the m= section in question may have existed for a long time.