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/san.cpp

@@ -299,7 +299,7 @@ const string line_to_san(const Position& pos, Move line[], int startColumn, bool
   string moveStr;
   size_t length = 0;
   size_t maxLength = 80 - startColumn;
-  Position p(pos);
+  Position p(pos, pos.thread());
 
   for (int i = 0; line[i] != MOVE_NONE; i++)
   {