Cleanup and optimize Position::has_mate_threat()

Cleanup and optimize Position::has_mate_threat()

There is a functional change because we now skip
more moves and because do_move() / undo_move() is
well known to be not reversible we end up with a
change in node count, although there is actually
no change but a bit speed up.

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