Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

struct AudioOutput should not know or care about multi-threading.

Fixes build failure on OS X, closes #44.  With the other plugins,
that's not critical, because those use the AudioOutputWrapper, which
hides this problem.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

Eliminate dependency on io_thread_get().

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