Use SearchStack to pass allowNullmove

Also renamed allowNullmove in skipNullMove to reverse
the logic so that the field is initialized to 0 (false)
instead of 1 (true).

No functional change also with faked split.

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

src/search.h

@@ -58,6 +58,7 @@ struct SearchStack {
   Move killers[KILLER_MAX];
   Depth reduction;
   Value eval;
+  bool skipNullMove;
 
   void init(int ply);
   void initKillers();