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.

Do error reporting with GLib's GError library.

Renamed functions.

Renamed functions and variables.

Moved code which deals with all audio outputs at once into a separate
library.

Export the "g_playlist" variable, and pass it to all playlist
functions.  This way, we can split playlist.c easier into separate
parts.  The code which initializes the singleton variable is moved to
playlist_global.c.

Don't mention the program name in the error message.

When the update thread is started before MPD has forked (for
daemonization), it is killed, because threads do not survive a fork().
This induces an inconsistent state where MPD won't start any update
thread at all, because it thinks the thread is already running.

Reimplemented the legacy mixer configuration: copy the deprecated
configuration values into the audio_output section.  Don't configure
the mixers twice (once for the audio_output, and a second time for the
legacy values).

This requires volume_init() to be called before initAudioDriver().

All config_get_block_*() functions should accept constant config_param
pointers.

Don't declare and export variables specific to stored playlists in
playlist.c/playlist.h.

Start the decoder thread when the player thread starts.  The decoder
thread is already stopped by the player thread.

"Stickers" are pieces of information attached to existing MPD objects
(e.g. song files, directories, albums).  Clients can create arbitrary
name/value pairs.  MPD itself does not assume any special meaning in
them.

This way, we don't have to pass the full "Options" object to
daemonize().

The constructor/destructor functions parse and free the configuration
properly.  This way, we don't have to load the pid file path more than
once.

The state_file library should manage its own regular saves.

The constructor/destructor interface should hide the functions
read_state_file() and write_state_file().

config_get_path() is an simpler interface than parseConfigFilePath().

Fetch the configuration variables buffered_chunks and
buffered_before_play just when they are needed.

Moved changeToUser(), cleanUpPidFile(), killFromPidFile() to
daemon.c.  These are daemonization functions.

Without a music_directory, MPD is an excellent streaming client.

Don't include conf.h in database.c.

time(NULL) shows the wrong results when the machine's clock is
changed.

Renamed functions and types.

Renamed functions, types, variables.

AC_CHECK_HEADERS defines HAVE_LOCALE_H, so we don't have to manually
define HAVE_LOCALE.

Make db_load(), db_save() and db_check() return bool instead of int.

For updating the database, directory_update_init() should be called
explicitly.

Start the daemon after --create-db.  This makes --create-db a flag
which discards the old database and starts with a fresh one.

There is only one location using PIPE_EVENT_SIGNAL: to synchronize
player_command() with player_command_finished().  Use the "notify"
library instead of the event_pipe here.

Make the event_pipe (formerly main_notify) send/receive a set of
events, with a callback for each one.

The default event PIPE_EVENT_SIGNAL does not have a callback.  It
is still there for waking up the main thread, when it is waiting for
the player thread.

Continuing the previous patch.

We are going to migrate away from the concept of notifying the main
thread.  There should be events sent to it instead.  This patch starts
a series to implement that.