Use a listener interface instead of GlobalEvents.

Another big chunk of code for multi-player support.

Replaces GLib's g_str_has_prefix().

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

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.

Flush the hardware volume cache, and send the MIXER idle event.  This
allows mixer plugins to detect volume changes.

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.

Parse the state file line by line, let each subsystem probe a line.
Only the playlist_state code gets the FILE pointer to read the
following lines.

Do all the software volume stuff inside each output thread, not in the
player thread.  This allows one software mixer per output device, and
also allows the user to configure the mixer type (hardware or
software) for each audio output.

This moves the global "mixer_type" setting into the "audio_output"
section, deprecating the "mixer_enabled" flag.

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 top-level "mixer_device" and "mixer_control" options have been
deprecated by MPD 0.15, and it's safe to remove them in MPD 0.16.

Renamed all remaining CamelCase functions.

Make gcc warn us if we add a new mixer type, and forget to add a new
"case" line.

Don't use CPP macros when you can use C enums.

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).

Return true/false on success/failure, instead of 0/-1.  Pass
true/false instead of 1/0 for the "rel" boolean parameter.

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.

On some hardware, reading the mixer value from hardware is an
expensive operation, and MPD has to do it for every client.  Throttle
access to the hardware, cache the result for one second.