Failure must be indicated by returning false.

This is necessary to avoid an assertion failure in UpdateQueuedSong()
after LockSeek() has failed.

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.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

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

For simpler seeking within current song.

Use a shortcut in playlist_seek_song(), don't call
playlist_play_order() because that would reset the "paused" state.

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.

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.