Another big chunk of code for multi-player support.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).

Replaces GLib's GError.

This fixes a deadlock bug introduced by 18076ac9.  After all, the
second Cond was necessary.

The problem: two threads can wait for a signal at the same time.  The
player thread waits for the output thread to finish playback.  The
main thread waits for the player thread to complete a command.  The
output thread finishes playback, and sends a signal, which
unfortunately does not wake up the player thread, but the main
thread.  The main thread sees that the command is still not finished,
and waits again.  The signal is lost forever, and MPD is deadlocked.

This commit reimplements the core of the "single" mode.  Instead of
doing the detection in the playlist code from the outside, it is moved
to the player thread, which gets a new option called "border_pause".
It will now pause playback exactly at the beginning of the new song,
making the feature more reliable.

Now that the player thread knows what will happen, it can suppress
cross-fading.

Fixes mantis tickets 0003055 and 0003166.

Make sure the player "owns" the next_song object, so nobody else can
free it.

Rewrite of the pc_get_error_message() function, now using a GError
object instead of the complicated "errored_song" attribute.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

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

Our use of the "volatile" keyword was wrong from the start, and now
that we have proper locking, we can safely remove all of them.

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.

With these methods, an output plugin can allocate some global
resources only if it is actually enabled.  The method enable() is
called after daemonization, which allows for more sophisticated
resource allocation during that method.

Tracking the "elapsed" time from the chunks which we have sent to the
output pipe is very imprecise: since we have implemented the music
pipe, we're sending large number of chunks at once, giving the
"elapsed" time stamp a resolution of usually more than a second.

This patch changes the source of this information to the outputs.  If
a chunk has been played by all outputs, the "elapsed" time stamp is
updated.

The new command PLAYER_COMMAND_REFRESH makes the player thread update
its status information: it tells the outputs to update the chunk time
stamp.  After that, player_control.elapsed_time is current.