Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

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.

Fixes regression from commit e96779de.

Leave that to the caller, allowing it to modify multiple items at a
time.

The check for unsigned underflow is fragile and unreliable.

Make sure the queue "owns" all song objects, so nobody else can free
them.

Sorts remaining songs by priority.  This can be used for the
much-demanded "queue feature".

Check consume mode in queue_next_order(), because the current song
would be deleted as soon as it's finished; it cannot be played again.

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.

Renamed idToPosition.

Consume mode removes each song played

When single mode is enabled, after current song it stops playback,
or it replay same song if repeat mode is activated.

The smartstop feature is a way to tell mpd to stop playing after
current song.
This patche provides:
- 'state' command returns 'smartstop' state (1 or 0)
- 'smartstop' can activate or not the smartstop state
- when song is terminated, mpd stops playing and smartstop is set to 0