functions returning sets

Well, I'm on my way to implement what was discussed on list before.

I am doing it the way Karel and Jan suggested: creating a
pg_class/pg_attribute tuple[s] for a function that returns a setof.

I have a special RELKIND_FUNC for parser, and it seems to go through fine,
and the final query plan has 'Seq Scan on rel####', which I think is a
good sign, as the the function should pretend to be a relation.

Now, more interesting question, what's the best way to interface ExecScan
to function-executing machinery:

Options are:

1) Create a special scan node type, T_FuncSeqScan and deal with it there.

2) Keep the T_SeqScan, explain to nodeSeqScan special logic when dealing
with RELKIND_FUNC relations.
(I prefer this one, but I would like a validation of it)

3) explain to heap_getnext special logic.