Rework pg_dump namespace search criteria so that dumping of user objects

Rework pg_dump namespace search criteria so that dumping of user objects
having names conflicting with system objects will work --- the search
path is now user-schema, pg_catalog rather than implicitly the other way
around.  Note this requires being careful to explicitly qualify references
to system names whenever pg_catalog is not first in the search path.
Also, add support for dumping ACLs of schemas.