Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

For simplicity, the MPD core should not have to deal with packing.  It
is rarely used, and those plugins that need it should use the
pcm_export library instead.

pcm_buffer_get() cannot ever return NULL.

Silently choosing default is misleading and can cause hours of investigation.
It's better to fail immediately telling user what is wrong with config.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Move the "extern" declarations from output_list.c, for more type
safety.

Device can be specified either by magic index (starting with 0)
or by device name.

If no device is available test_default_device returns false.

Win32 has many audio APIs. New name is slightly more correct.