[CM] multichannel with osx-10.2-snd thru motu828

[Bill Schottstaedt]

I looked at portaudio's code for this, and it appears that
the Motu card is a special case in this regard.
So, I've added a check for that in audio.c, and it's
worth a try rebuilding sndplay or snd with the new audio.c.
In Snd, you can find out what it thinks the max chan is
via

(define vals (make-vector 4))
(mus-audio-mixer-read mus-audio-default mus-audio-channel 4 vals)
(vector-ref vals 0)

Also, in the new code if the two max-chan values differ, you'll
see a mild complaint about that in mus-audio-describe.