Retire ss->bestMove

And introduce SPlitPoint bestMove to pass back the
best move after a split point.

This allow to define as const the search stack passed
to split.

No functional change.

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

src/search.h

@@ -39,7 +39,6 @@ struct Stack {
   int ply;
   Move currentMove;
   Move excludedMove;
-  Move bestMove;
   Move killers[2];
   Depth reduction;
   Value eval;