Rewrite Position::from_fen()

Complete rewrite the function and extend compatibility
also to X-FEN notation for Chess960.

We are now able to read standard FEN, Shredder-FEN and X-FEN.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/position.h

@@ -294,6 +294,7 @@ private:
   void put_piece(Piece p, Square s);
   void allow_oo(Color c);
   void allow_ooo(Color c);
+  bool set_castling_rights(char token);
 
   // Helper functions for doing and undoing moves
   void do_capture_move(Key& key, PieceType capture, Color them, Square to, bool ep);