Restructure AclItem representation so that we can have more than eight

Restructure AclItem representation so that we can have more than eight
different privilege bits (might as well make use of the space we were
wasting on padding).  EXECUTE and USAGE bits for procedures, languages
now are separate privileges instead of being overlaid on SELECT.  Add
privileges for namespaces and databases.  The GRANT and REVOKE commands
work for these object types, but we don't actually enforce the privileges
yet...