[mythtv] output mutex lock

[Brent Hills]

I've been taking a look at mythmusic on Fedora.

Using the UI functions like pause() in PlaybackBox::pause() hangs the
frontend for extended periods trying to acquire the output mutex lock.

MMAudioOutput::run() keeps the mutex locked for all but very brief
periods seemingly making it difficult for the other thread to acquire.

I looked at the mythtv audio output but its using the posix api instead
of the QT one and the code is structured quite different.

Anyone more familiar with threads have a suggestion on how to change the
code here and make it more friendly to NPTL?

Brent.