Playlist file name first, to be consistent with the other functions in
this library.

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 GError.

.. instead of rolling own classes.

The "bool" type is built-in.

Improve error reporting and handling.  command.c gets the new function
print_error(), which sends a GError to the client.

Reduce header dependencies.

These fixes were mostly generated with `codespell' [0] and manually
reviewed.

[0] http://git.profusion.mobi/cgit.cgi/lucas/codespell/

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Don't declare and export variables specific to stored playlists in
playlist.c/playlist.h.

On some systems, the time_t data type was not present, because
stored_playlist.h didn't include the time.h header directly.