Automate the life cycle, making the code safer.

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 GLib's g_str_has_prefix().

Overload the name.

Replaces struct playlist_provider.

Replace g_strchug() calls with a cheaper implementation.

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.

This new plugin parses extm3u files.  Files without the "#EXTM3U"
header are still parsed by the plain old "m3u" plugin.