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

Replaces GLib's GError.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The special-purpose function is used for saving/restore the software
volume control to the state file.

Converted the range checks in volume_level_change() to assertions.
Changed all volume types to "unsigned", expect for those which must be
able to indicate error (-1).

Since the "volume" command has been removed, nobody uses relative
volumes anymore.

The mixer core library is now responsible for creating and managing
the mixer object.  This removes duplicated code from the output
plugins.

Don't dump the volume of all mixers.

mixer_control.h should provide the functions needed to manipulate a
mixer, without exposing the internal mixer API (which is provided by
mixer_api.h).

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.

Moved code which deals with all audio outputs at once into a separate
library.

Moved everything related to saving or loading the playlist from/to the
state file to playlist_state.c.

In generic PCM functions allowing all sample formats, pass a void
pointer instead of a char pointer.

Moved the software mixing code (used by crossfading) to a separate
library.

Don't use libc's rand() function, because it is slow.  Our own trivial
linear congruential generator is good enough for dithering.

redirect_stdin() is a daemonization function, and disconnecting from
the standard input is always a good idea for MPD.

Prepare for the migration to the GLib option parser, which uses
gboolean for flags.

"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.

When there are standardized headers, use these instead of the bloated
os_compat.h.

Similar to decoder_control.c, output_control.c will provide functions
for controlling the output thread (which will be implemented later).

This function is declared, but is neither used nor implemented.

Instead of having to register each output plugin, store them
statically in an array.  This eliminates the need for the List library
here, and saves some small allocations during startup.

Also rename AudioOutputPlugin to struct audio_output_plugin, and use
forward declarations to reduce include dependencies.

Just like decoder_api.h, output_api.h provides the audio output API
which is used by the plugins.