Modify nodeAgg.c so that no rows are returned for a GROUP BY

Modify nodeAgg.c so that no rows are returned for a GROUP BY
with no input rows, per pghackers discussions around 7/22/99.  Clean up
a bunch of ugly coding while at it; remove redundant re-lookup of
aggregate info at start of each new GROUP.  Arrange to pfree intermediate
values when they are pass-by-ref types, so that aggregates on pass-by-ref
types no longer eat memory.  This takes care of a couple of TODO items...