The old struct config_param remains only for top-level string options.

Require the version 2 which has a more sane API.

For free().

Replaces the method Close().

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

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.

The MPD core takes care for checking the scheme.

Merge some duplicate code.