aout: make drain asynchronous

aout_DecDrain() is now asynchronous, except for modules still implementing
aout->drain (only during the transition).

Add aout->drain_async(). Aout modules should implement drain_async
instead of drain, and call aout_DrainedReport() to report that the
stream is drained.

Aout modules not implementing any drain functions will still rely on
aout_TimeGet() to detect the end of the stream.