ALSA support for uniform audio. ALSA and OSS support now shares the

threading work, which is split into a collecter and a player thread
communicating via a ring of buffers.

New uaudio_set_format() allows caller to declare what the sample format
will be, giving us flexibility to do things other than 16/44100/2
(although not tested with other formats).

Core audio uniform audio support now supports more sample formats, and
checks that the channel count and rate are right.  It doesn't
(currently) attempt to ask for different channel counts or rates.