UPDATE using sub selects

Hi,

As per discussion on -hackers, a patch which allows updates to use
subselects is attached with this mail.

As per discussion with Tom, I have adopted the following approach:

* Introduce ROWEXPR_SUBLINK type for subqueries that allows multiple column
outputs.
* Populate the targetList with PARAM_SUBLINK entries dependent on the
subselects.
* Modify the targets in-place into PARAM_EXEC entries in the make_subplan
phase.

The above does not require any kluges in the targetList processing code path
at all.

UPDATEs seem to work fine using subselects with this patch. I have modified
the update.sql regression test to include possible variations .

No documentation changes are present in this patch.
Feedback, comments appreciated.

Regards,
Nikhils
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