Print reversible plies done in stead of plies to go

Print reversible plies done in stead of plies to go

Printing the number of plies left to the draw-adjudication point was
confusing when the N-move rule was set to a non-standard value.
In stead we now print the number of plies since the last irreversible
move, so the user can compare it to both the engine claim point (100)
and the adjudication point he set (which might be beyond 100).