Obsolete class CloseSongEnumerator, which was a kludge.

Let the caller decide whether to strip.  Only remove \n and \r
(end-of-line markers).

Revert to the old API before commit e9e55b08, removing unnecessary
bloat.

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.

Replaces struct playlist_provider.

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.

The caller is responsible for verifying the song URI.

Based on this API, we will add parsers for EXTM3U, PLS, ASX, last.fm
radio and others.

There is no integration into the MPD core yet.  Right now, we have a
command line test program.  This is work in progress.