Always assume FEN in variant-fairy PGN game is initial position

Always assume FEN in variant-fairy PGN game is initial position

As variant fairy is a catch-all without a defined initial position,
the FEN tag n a PGN game for this variant must be considered an initial
position. So we copy over the FEN position to the initial one in this
case. Otherwise castling would always be thought illegal in fairy games
loaded from PGN.