"[L]itigation teaches that it isn't so much the question of whether directors should ever be a plan fiduciary -- accidentally or deliberately -- that is important, but rather the act of thinking logically in advance about who best in a company should have what roles with regard to a plan. Doing the latter not only protects against unanticipated litigation exposures, but also decreases the likelihood of litigation by increasing the probability that the plan will be in the hands of the executives best placed to run it well."