Add uaudio -- a USB audio device driver.

Add uaudio -- a USB audio device driver.

This driver actually works slightly better on -stable than on -current
(the system locks on detach on -current), so it should be MFC'd somewhat
sooner.

This driver currently points out a difficulty in the sound device framework.
The PCM unregister routine is allowed to refuse the detach if the device is
in use. In the case of a USB device, however, this unregistration is much more
mandatory in nature, since the device is *actually* gone when this call is
made. The sound subsystem really should not refuse an unregistration and
should take its own steps to reject further I/O. As a result, if you detach
a USB sound device while it is in use, you can expect a panic shortly
thereafter.

This device cannot currently record audio. Some routines are unwritten as
of yet in uaudio.c to support recording.

This device hangs my -current box on detach. I don't know why. This does
not happen on my -stable machine.

Obtained from:	Hiroyuki Aizu
MFC after:	2 weeks