RFC: time synchronised playback

This converts buffered playback to time synchronised playback.
-adds a ntpsender and a ntpreceiver thread to measure sender time offset
-tries to synchronise audio at the sync packets by adjusting the playback rate

The code is not quite ready for show time:
Regulation is twitchy
There are audible glitches maybe due to the above?
Time related function should probably be moved to timing.[ch]
What to do with sinks that do not provide delay information?

Anyway, I'd like some feedback from interested parties.