pipe: added music_buffer, rewrite 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.

src/player_control.c

@@ -30,8 +30,9 @@
 
 struct player_control pc;
 
-void pc_init(unsigned int buffered_before_play)
+void pc_init(unsigned buffer_chunks, unsigned int buffered_before_play)
 {
+	pc.buffer_chunks = buffer_chunks;
 	pc.buffered_before_play = buffered_before_play;
 	notify_init(&pc.notify);
 	pc.command = PLAYER_COMMAND_NONE;