Re: WIP - xmlvalidate implementation from TODO list

Hello there again, Jim!

Here's the v6 with the docs updated and some minor things. I have questions
about the predefined roles however.

The tab completion on psql was using the wrong OID type, so I took the
chance to change it to only complete unqualified XML schemas in
current_schemas with true.

I noticed I had added a change I made to run locally on pg_regress that
shouldn't be on the patch, so I reversed it (execl to execlp because of
NixOS search paths).

I was trying to figure out how to do the roles, and I have some thoughts on
it now. While I am not particularly against predefined roles, I think we
should be careful before committing to them in this form. As predefined
roles, they would effectively become part of a long-term public interface:
once they exist, we should then carry them forward and preserve their
semantics across releases, so adding a feature-specific role increases
permanent surface area (docs, tests, upgrade behavior, compatibility
expectations) and is harder to revisit later if the privilege model evolves
(which I think will make people mad, it's XML burden after all).

Also, a write-oriented role is not as straightforward as I thought with the
current ownership model (and that's me guessing here). Mutating DDL
behavior is still owner-driven (or superuser-driven) most of the places,
not purely ACL-driven, so a pg_write_xmlschemas role can look clearer at
first glance than it is in practice. To make that role fully consistent, we
may need broader policy decisions around what should be grantable versus
what should remain ownership-based, not just new role entries. Again, I am
not opposed to the idea, but these made me a bit anxious to simply add
them. What are your thoughts on these?

PS: Reattached patch 2 as v5 just to make it easier to get everything, but
nothing changed in there.

Regards,
Marcos Magueta.