Teach planner how to propagate pathkeys from sub-SELECTs in FROM up to

Teach planner how to propagate pathkeys from sub-SELECTs in FROM up to
the outer query.  (The implementation is a bit klugy, but it would take
nontrivial restructuring to make it nicer, which this is probably not
worth.)  This avoids unnecessary sort steps in examples like
SELECT foo,count(*) FROM (SELECT ... ORDER BY foo,bar) sub GROUP BY foo
which means there is now a reasonable technique for controlling the
order of inputs to custom aggregates, even in the grouping case.