Save threadID info in Position

This is the best place because when we split we do a
copy of the position and also threadID, once set in a
given position, never changes anymore.

Forbid use of Position's default and copy c'tor to avoid
nasty bugs in case a position is created without explicitly
setting the threadID.

No functional change.

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

src/benchmark.cpp

@@ -151,7 +151,7 @@ void benchmark(const string& commandLine) {
   {
       Move moves[1] = {MOVE_NONE};
       int dummy[2] = {0, 0};
-      Position pos(*it);
+      Position pos(*it, 0);
       cerr << "\nBench position: " << cnt << '/' << positions.size() << endl << endl;
       if (limitType == "perft")
       {