Bug #6593, extensions, and proposed new patch policy

Hackers,

Per bug #6593, REASSIGN OWNED fails when the affected role owns an
extension. This would be trivial to fix if extensions had support code
for ALTER EXTENSION / OWNER, but they don't. So the only back-patchable
fix right now seems to be to throw an error on REASSIGN OWNED when the
user owns an extension. (If anyone wants to claim that we ought to work
on a real fix that allows changing the owner internally from REASSIGN
OWNED, without introducing ALTER EXTENSION support for doing so, let me
know and I'll see about it.)

In HEAD we can do the more invasive fix of actually adding support code
for changing an extension's owner. And it seems to me that, going
further, we should have a policy that any ownable object type we add
must come with appropriate support for changing owner.

Thoughts?

--
Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>