Department of second thoughts: fix newly-added code in planner.c to make real

sure that DISTINCT ON does what it's supposed to, ie, sort by the full ORDER
BY list before unique-ifying.  The error seems masked in simple cases by the
fact that query_planner won't return query pathkeys that only partially match
the requested sort order, but I wouldn't want to bet that it couldn't be
exposed in some way or other.