In the spirit of 890192e9, this time add support for the things
living in the pg_default_acl catalog.  These are not really "objects",
but they show up as such in event triggers.

There is no "DROP DEFAULT PRIVILEGES" or similar command, so it doesn't
look like the new representation given would be useful anywhere else, so
I didn't try to use it outside objectaddress.c.  (That might be a bug in
itself, but that would be material for another commit.)

Reviewed by Stephen Frost.