Updated INSERT/UPDATE RETURNING

Here's the updated patch with DELETE RETURNING removed.  This isn't
really an issue because no one wanted DELETE RETURNING to begin with.

It is important to note that this patch is not yet ready to be
committed.  I still need to go through and run some more tests on it
but wanted to put it in the queue again and let ya know I've been
given time to make sure it gets in.

This patch includes:
- Code changes to core
- Code changes to PL/pgSQL
- Preliminary Documentation Updates (need to add to PL/pgSQL docs)
- Preliminary Regression Tests (need to add PL/pgSQL regressions)

There were a couple suggestions for sorta-kewl features like being
able to use INSERT/UPDATE RETURNING in a FOR loop, etc.  I may be able
to get those in if people really want it but I looked into it after
Neil mentioned it and IIRC, there are quite a few changes required to
support it.

Suggestions requested.

--
Jonah H. Harris, Software Architect | phone: 732.331.1300
EnterpriseDB Corporation            | fax: 732.331.1301
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