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.

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.

Fixes gcc warnings.

Wrap the locking code.

GLib has a built-in OOM handler.

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

Use these instead of exposing the internal roar_t struct.

Should fix build errors for Win32.