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/benchmark.cpp

@@ -125,7 +125,7 @@ void benchmark(int argc, char* argv[]) {
   {
       Move moves[1] = { MOVE_NONE };
       int dummy[2] = { 0, 0 };
-      Position pos(*it, 0);
+      Position pos(*it, false, 0);
       cerr << "\nBench position: " << cnt << '/' << positions.size() << endl << endl;
       if (valType == "perft")
       {