Fix ponder start

Fix ponder start

There was an inconsistency in starting ponder searches, because they occur
with the opposite side-to-move, or thus exchange wtime and btime. This was
solved in the old version by changing stm only after starting the ponder
search in response to an engine move. But for starting through 'hard'
there is no turn change. So the start routine has to swap the times by
itself, and the turn change was moved to before it to compensate.