new Group BY code

I hope that GROUP BY fixed.
Unfortunately, I can't get any errors under FreeBSD 
when using GROUP BY with aggregates. I only got core
for something like this: select y, z from tgrp group by y, z.
Two different funcs are used for these two cases and so 
please test new code on your platforms.
Please test with different -S XXX to check that all works
for both in-memory and on-disk sorts (you'll see pg_psort.xxxx
files in last case).

New code are for 6.3. This is not patch (sorry, I lost orig version
of execnodes.h...) and has to be un-tarred in src dir.

If someone hasn't 6.3 - ok, I'll post patch for 6.2.1 tomorrow.

CVS updated.

Vadim