Make aucat audio and midi backends share the same code to communicate
with the server. As we're at it use the same protocol for midi and
audio. Now, both audio and midi code use the same SNDIO_DEBUG

Make aucat audio and midi backends share the same code to communicate
with the server. As we're at it use the same protocol for midi and
audio. Now, both audio and midi code use the same SNDIO_DEBUG environment
variable to turn on/off DPRINTF's.

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Clean up function naming: use sio_<backend>_ prefix for private
audio-related functions and put them in files named sio_<backend>.c
No behaviour changes.