'api' configuration command now uses uaudio. The list of APIs is only

available in server processes (since it currently drags in the API
implementations which in turn drags in external audio support
libraries, which extends the dependencies of the command-line client
unacceptable).

disorder-speaker now uses uaudio, abolishing the various speaker-*.c
API-specific files.

uaudio now handles volume control as well as playback, abolishing
mixer.h et al.

"rtp" is the preferred name for the RTP API.  "network" is retained
for backward compatibility.

Lightly tested - works for local play on a Mac.