While poking around in the aggregate logic, I noticed that commit
8ed3f11b broke the logic in nodeAgg.c that purports to detect nested
aggregates, by moving initialization of regular aggregate argument
expressions out of the code segment that checks for that.

You could argue that this check is unnecessary, but it's not much code
so I'm inclined to keep it as a backstop against parser and planner
bugs.  However, there's certainly zero value in checking only some of
the subexpressions.

We can make the check complete again, and as a bonus make it a good
deal more bulletproof against future mistakes of the same ilk, by
moving it out to the outermost level of ExecInitAgg.  This means we
need to check only once per Agg node not once per aggregate, which
also seems like a good thing --- if the check does find something
wrong, it's not urgent that we report it before the plan node
initialization finishes.

Since this requires remembering the original length of the aggs list,
I deleted a long-obsolete stanza that changed numaggs from 0 to 1.
That's so old it predates our decision that palloc(0) is a valid
operation, in (digs...) 2004, see commit 24a1e20f.

In passing improve a few comments.

Back-patch to v10, just in case.