has_language_privilege, has_schema_privilege to let SQL queries test
all the new privilege types in 7.3.  Also, add functions pg_table_is_visible,
pg_type_is_visible, pg_function_is_visible, pg_operator_is_visible,
pg_opclass_is_visible to test whether objects contained in schemas are
visible in the current search path.  Do some minor cleanup to centralize
accesses to pg_database, as well.