Let material probing to access per-thread table

It is up to material (and pawn) table look up
code to know where the per-thread tables are,
so change API to reflect this.

Also some comment fixing while there

No functional change.

src/tt.cpp

@@ -17,7 +17,7 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include <cstring>
+#include <cstring>   // For std::memset
 #include <iostream>
 
 #include "bitboard.h"