Commit f78cddb4 introduced a regression: after a song was moved, the
order array was not updated (in random mode). This caused MPD to
think the "current" song has changed when you moved something to the
position of the current song.
The function shuffles the virtual order of songs, but does not move
them physically. This is used in random mode.
The new function replaces playlist.c's randomizeOrder() function,
which was aware of playlist.current and playlist.queued. The latter
is always -1 anyway, and the former as preserved by the caller, by
converting playlist.current to a position, and then back to an order
number.
