Turning off cluster patch

Hi,

The attached patch adds a new command:

ALTER TABLE [ONLY] foo SET WITHOUT CLUSTER;

I would probably have preferred the DROP CLUSTER syntax, but it doesn't
seem easy to get working without shift/reduce problems.

It has support for inheritance.

I have also fixed the previously detailed security errors:

* Ownership is now checked
* Can now not cluster system catalogs
* Checks that what you are clustering is actually a table

And it fixes a related bug in the SET WITHOUT OIDS implementation:

* Checks that what you are dropping OIDS from is actually a table

I have included a documentation update and a regression test.

Chris