superuser unable to modify settings of a system table

On PG 8.4.4 I am unable to set per-table autovacuum setting for the
pg_listener table. Being a superuser, I'd expect Postgres to allow me to do
that.

postgres=# select version();

version
-----------------------------------------------------------------------------------------------------------
PostgreSQL 8.4.4 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 4.3.2
20081105 (Red Hat 4.3.2-7), 32-bit
(1 row)

postgres=# select user;
current_user
--------------
postgres
(1 row)

postgres=# alter table pg_listener set (autovacuum_vacuum_threshold=100);
ERROR: permission denied: "pg_listener" is a system catalog
postgres=# select * from pg_user;
usename | usesysid | usecreatedb | usesuper | usecatupd | passwd |
valuntil | useconfig
----------+----------+-------------+----------+-----------+----------+----------+-----------
postgres | 10 | t | t | t | ********
| |
(1 row)

Regards,
PS: RhodiumToad prompted me to post here.
--
gurjeet.singh
@ EnterpriseDB - The Enterprise Postgres Company
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