Fixes #165.

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

This commit changes a minor queue priority design to something which
makes a little bit more sense.

Previously, a song that had already been played would only be
re-enqueued if its priority had just been raised above the current
song's.  This means that if it was already above, it was not
re-enqueued.  That is a surprising behavior, because users expect a
song to be played when its priority is raised.

Now the song is always re-enqueued if its priority is raised (and
above the current song's - no matter if it has already been above
before).

 https://bugs.musicpd.org/view.php?id=4592

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.