This will allow "addid \"song_url\" <pos>" to atomically insert a
song at any given playlist position.

If the add succeeds, but the actual movement fails (due to
invalid position), then the song_id will be deleted before
the command returns back to the client, and the client
will get an error response.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7151 09075e82-0dd4-0310-85a5-a0d7c8717e4f