Make all uses of MusicChunk safe.

This tiny amount of overhead allows omitting the MusicBuffer in
Clear().

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.

Always assert that the audio format of the new chunk is valid.

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.

In !NDEBUG, remember which audio_format is stored in every chunk and
every pipe.  Check the audio_format of every new data block appended
to the music_chunk, and the format of every new chunk appended to the
music_pipe.

Added two assertions.

The value of music_chunk.next is undefined for a chunk returned by
music_pipe_shift().  For more pedantic debugging, poison the reference
before returning the chunk.

music_pipe_peek() is similar to music_pipe_shift(), but doesn't remove
the chunk.  This allows it to be used with a "const" music_pipe.

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.

Added music_pipe_allocate(), music_pipe_push() and
music_pipe_cancel().  Those functions allow the caller (decoder thread
in this case) to do its own chunk management.  The functions
music_pipe_flush() and music_pipe_tag() can now be removed.

Moved some code from music_pipe_write() and music_pipe_expand().  Only
music_chunk.c should access the music_chunk internals.

The function is unused, since decoder_api() uses music_pipe_write() /
music_pipe_expand() now.

This new API gives the caller a writable buffer to the music pipe
chunk.  This may allow the caller to eliminate several buffer copies,
because it may manipulate the returned buffer, until it calls
music_pipe_expand().

The chunk index passed to successor() must be valid, i.e. it must be
smaller than num_chunks.

When a CANCEL command is received, the player should drop all chunks
of the next song.  Added new funciton music_pipe_chop() which is used
for that.

Provide a debug function which asserts on the sample format of all
chunks.  Call this function in each iteration of the player main loop.