Adds missing checks to several plugins.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Eliminate dependency on io_thread_get().

The old struct config_param remains only for top-level string options.

Replaces GLib's g_strsplit().

Make it "const".

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

Replaces GLib's GError.

Make sure we get a UTF-8 encoded string.

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.

Version 1.0.0 of the libao library added a new field to the
ao_sample_format struct.  It is a char * named matrix.  When
an ao_sample_format is allocated on the stack, this field contains
garbage.  The proper course is to insure that is initialized to NULL.
NULL indicates that we do not want any mapping.
The struct is now initialized using a static initializer, and this
technique is compatible with all known versions of libao.