Respond immediately to ponder on/off commands

Respond immediately to ponder on/off commands

Rather than just setting the flag that causes a ponder search to start
after the engine moves, we start the search on receiving the 'hard' command.
When a ponder search is in progress, it is stopped on receiving 'easy'.
This requires saving a ponder move even with ponder off, (because it might
be switched on later), and more strict clearing of the move beyond the last
one, toprevent it can be accidentally used as ponder move (e.g. after undo).