Bugfix for #492: Audio output on Windows useing SPDIF for AC3 / DTS

But its still very time critical, because paout gets sometimes empty
so that aout_OutputNextBuffer(..) doesn't deliver new audiobuffers,
sometimes it happens - that these buffers arrive very late for output.
Reasons: -changed CPU load, (extra running application, also fast forward
seeking and and jumping may lead to this situation.)

Enhancement #897: added an option to let the user  choose is prefered audio device. (needs restart of VLC to get applied)