Use the "active" flag in PrepareSockets() instead of LockHasError().

Prepare to make the "active" attribute thread-safe.

From: Christian Kröner <ckroener@gmx.net>

This just copies the necessary bits and pieces from the ALSA plugin and applies them to OSXOutput based on dop config setting. It only changes the OSXOutput plugin as needed for DoP (further changes to support additionally e.g. integer mode or setting the physical device mode require rather a complete rewrite of the output plugin).

Fortunately the Core Audio API is by default bit perfect and supports DoP with minimal changes (setting the sampling rate accordingly after ensuring that the physical mode supports at least 24 bits per channel seems to be enough). This was tested on an Amanero Combo384 device hooked up to a ES9018 DAC.

USAGE (try only on DACs that support DoP):
- Add dop "yes" option to mpdconf
- Be sure to set at least 24bits per channel before playing some DSD file (using Audio-MIDI-Setup)
- Based on the dop setting, MPD will change the sample rate as required and output DoP signal to the DAC
- Hog mode is recommended to ensure that no other program will try to mix some output with the DoP stream (resulting in bad noise)
- Alternatively set the default output device to another device (e.g. the built-in output) to avoid having other audio interfere with DSD playback

The PulseOutput needs to be "enabled" before WaitConnection() may be
called.

Closes #207

Fixes deadlock due to mutex double lock.

Convert the fill_buffer to a class method, to allow accessing the private method.

Also remove the leftover Wrapper stuff.

All it does is "sometimes" close the mixer, but that is also done in
the destructor.

Fixes shutdown memory leak.

This is implicitly done by the destructor.

Fixes shutdown memory leak.

The output plugin shall decide whether to insert silence or do nothing
at all.  The ALSA output plugin has already implemented this.
Inserting silence is not necessary or helpful for some plugins, and
may even hurt them (e.g. "recorder").

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.