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/pawns.h

@@ -80,7 +80,7 @@ struct Entry {
 typedef HashTable<Entry, 16384> Table;
 
 void init();
-Entry* probe(const Position& pos, Table& entries);
+Entry* probe(const Position& pos);
 
 } // namespace Pawns