Add the debug-only flag "virgin" which gets checked by assert()
calls.  Fixes assertion failures when using zeroconf/avahi.

Using a variable-size struct with embedded string is not worth the
trouble here.  There are not so many Directory objects.

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.

Don't create a temporary Song object in handle_lsinfo().  Instead,
print all tags while parsing the remote file.

Don't pass a GError** to g_key_file_get_X().  We don't need to dispose
something we didn't request in the first place.

By accident, this declared a global variable instead of adding the
"packed" attribute.

Explicit attribute initialization.

The MPD core takes care for checking the scheme.