More abstraction for decoder_control.pipe.

Adds mixrampdb and mixrampdelay commands.  Reads MIXRAP_START and
MIXRAMP_END tags from FLAC files and overlaps instead of crossfading.

Don't access decoder_control attributes directly.

Asynchronous decoder startup is gone, and we don't need to check
command==DECODE_COMMAND_START anymore.

These two variables are redundant, we need only one of them.

The START command returns without blocking; we don't need the
asynchronous decoder start anymore.

They are just informational.

Allocate a decoder_control object where needed, and pass it around.
This will allow more than one decoder thread one day.

Use GMutex/GCond instead of the notify library.  Manually lock the
player_control object before accessing the protected attributes.  Use
the GCond object to notify the player thread and the main thread.

Replace decoder_control.notify with decoder_control.mutex and
decoder_control.cond.  Lock the mutex on all accesses to
decoder_control.command and decoder_control.state.

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.

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

Minimize header dependencies, again.

decoder_control.thread contains the handle of the decoder thread, or
NULL if the decoder thread isn't running.

When MPD exits, it should manually free all resources in use, to allow
easy memory leak debugging.  Make the decoder thread terminate during
that.

The player did not care about the exact error value, it only checked
whether an error has occured.  This could fit well into
decoder_control.state - introduce a new state "DECODE_STATE_ERROR".

The variable "next_song" is already protected by a memory barrier.
"total_time" is not important for synchronization, and we don't need
"volatile" here.

Renamed variables and functions.

.. and rename dc.audioFormat to dc.in_audio_format.  The music pipe
does not need to know the audio format, and its former "audioFormat"
property indicated the format of the most recently added chunk, which
might be confusing when you are reading the oldest chunks.

"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.

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

Typedefs shouldn't be used, use the bare struct names instead.

Again, a data type which can be forward-declared.

Do full C99 integer type conversion in all modules which were not
touched by Eric's merged patch.

Destroy the mutex when it is not used anymore.

Get rid of CamelCase, and don't use a typedef, so we can
forward-declare it, and unclutter the include dependencies.

It should be obvious in which thread or context a function is being
executed at runtime.  The code which was left in decode.c is for the
decoder thread itself; give the file a better name.

The decoder plugins need this type, so export it in the public API.
This allows is to remove "decode.h" from "decoder_api.h", uncluttering
the API namespace some more.

Now that "dc" is available here, we don't have to pass it to
decoder_is_idle() and decoder_is_starting() anymore.

Unfortunately, we have to pass the DecoderControl pointer to these
inline functions, because the global variable "dc" may not be
available here.  This will be fixed later.

The source "decoder_control.c" provides an API for controlling the
decoder.  This replaces various direct accesses to the DecoderControl
struct.

Move code which runs in the player thread to player_thread.c.  Having
a lot of player thread code in decode.c isn't easy to understand.

Some decoder commands are implemented in the decoder plugins, thus
they need to have an API call to signal that their current command has
been finished.  Let them use the new decoder_command_finished()
instead of the internal dc_command_finished().

We want to expose the AudioFormat structure to plugins; remove some
clutter by moving its declaration to a separate header file.