Fixes mixer lag (http://bugs.musicpd.org/view.php?id=4425).

Another big chunk of code for multi-player support.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

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.

With these methods, an output plugin can allocate some global
resources only if it is actually enabled.  The method enable() is
called after daemonization, which allows for more sophisticated
resource allocation during that method.

If nothing has changed since the last save, don't save the state
file.  Saving will spin up the hard drive, which is undesirable on
hosts where MPD is idling in background.

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

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.

When an audio output device is disabled, also disable its mixer.

time() is not a monotonic timer, and MPD might get confused by clock
skews.  clock_gettime() provides a monotonic clock, but is not
portable to non-POSIX systems (i.e. Windows).  This patch uses GLib's
GTimer API, which aims to be portable.

The output_command library provides a command interface to the audio
outputs.  It assumes the input comes from an untrusted source
(i.e. the client) and verifies all parameters.

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.