Re: INS/UPD/DEL RETURNING for 8.2

This might tie into something that was bothering me about Jonah's
first-cut patch, which was that he was introducing special cases into
places where it didn't seem real appropriate (like printtup.c).  I
wonder if we should rejigger the representation of Query so that a
FOO-RETURNING command actually *is* a SELECT in some sense, so that
there's no need for special cases.

I'm a bit fuzzy about how this would work exactly --- you still need to
keep track of two targetlists it seems --- but it's worth thinking
about.  I've had a bee in my bonnet for literally years about the fact
that INSERT/SELECT really needs two levels of targetlist, as does UNION.
Maybe if we thought a little bit larger we could clean up all of that
messiness at one stroke.