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

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

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

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.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Use config_get_block_string("name") instead of audio_output_get_name().

If an output plugin requires config.h, it should include it directly.

The method control() is too complicated, and overengineered.  Replace
it with two trivial functions: get_volume() and set_volume().

If we move the plugin struct to a separate header, we don't have to
include the big fat output_api.h everywhere.

Now that the output_command enum isn't exposed to output plugins
anymore, we can hide its definition within output_internal.h.

Including "../config.h" breaks on some systems.

Move the "while" loop which checks for commands to the caller
ao_pause().  This simplifies the pause() method, and lets us remove
audio_output_is_pending().

The macro is unused.

The function is only used by the MVP output plugin, and this one call
is wrong.

Those have been superseded by the new legacy configuration code.

All config_get_block_*() functions should accept constant config_param
pointers.

Renamed functions, types, variables.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.