Pass evalMargin through SearchStack as eval

It has more sense to treat the two evaluation metrics
in the same way.

As a side effect now we use the correct eval margin when
pruning in a SplitPoint node.

No functional change in single thread.

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

src/search.h

@@ -57,6 +57,7 @@ struct SearchStack {
   Move killers[2];
   Depth reduction;
   Value eval;
+  Value evalMargin;
   bool skipNullMove;
   SplitPoint* sp;
 };