Beware, this commit was not tested.  I don't have OS X, but I want to
prepare an API change.

Eliminate all those "goto"s and make the function exception-safe.

This further optimize the performance. 

This is now working properly with a conditional variable bug being fixed (8bbfb5cd).

do not hog system device.

which ensures mpd do bit perfect playback on OS X

Closes https://bugs.musicpd.org/view.php?id=4537 .

Removed the 'cancel' function because it violates 'single producer,
single consumer'.

In e068d62a I added code that zeros the remainder of the output buffer
if there are not enough input frames available. I have now learned
that we can signal to the caller of the render callback how much data
is in the output buffers. In practice, the input buffer is so large
that it does not matter so much how we handle input buffer underruns,
but I suppose that saying 'there is no data' is better than 'here is
some silence for you'.

The old struct config_param remains only for top-level string options.

Make it "const".