Another attempt at fixing Chess960

Keep the isChess960 flag inside Position so that is
copied with the Position, but esplicitly highlight the
fact that a FEN string has not enough information to detect
Chess960 in general case. To do this add a boolean argument
isChess960 to from_fen() function so to self document this
shortcoming of FEN notation.

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

src/material.cpp

@@ -407,7 +407,7 @@ Key EndgameFunctions::buildKey(const string& keyCode) {
     }
     fen += char(8 - keyCode.length() + '0');
     fen += "/8/8/8/8/8/8/8 w - -";
-    return Position(fen, 0).get_material_key();
+    return Position(fen, false, 0).get_material_key();
 }
 
 const string EndgameFunctions::swapColors(const string& keyCode) {