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/evaluate.cpp

@@ -562,7 +562,7 @@ namespace {
         }
 
         // Special extra evaluation for bishops
-        if (Piece == BISHOP)
+        if (Piece == BISHOP && pos.is_chess960())
         {
             // An important Chess960 pattern: A cornered bishop blocked by
             // a friendly pawn diagonally in front of it is a very serious