Nasty security bug with clustering

No check is performed for being a superuser, the table owner or that it
is a system table when marking an index for clustering:

usa=> alter table pg_class cluster on "pg_class_oid_index";
ALTER TABLE
usa=> select oid from pg_class where relname='pg_class_oid_index';
oid
-------
16613
(1 row)

usa=> select * from pg_index where indexrelid=16613;
indexrelid | indrelid | indkey | indclass | indnatts | indisunique |
indisprimary | indisclustered | indexprs | indpred
------------+----------+--------+----------+----------+-------------+--------------+----------------+----------+---------
16613 | 1259 | -2 | 1989 | 1 | t | f
| t | |
(1 row)

Note how I managed to mark as clustered an index on a system catalog as
a non-superuser...

Chris