Restore pliesFromNull counter

It is not equivalent because the check for the
50 moves rule get badly affected.

// Draw by the 50 moves rule?
if (st->rule50 > 100 || (st->rule50 == 100 && !is_check()))
    return true;

So we _really_ need two counters.

Thanks to Joona and Tord to be patience with a silly guy ;-)

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