CLUSTER all tables

Hello,

This is a first attempt at CLUSTER ALL. It works in every way I've
tested, but maybe I'm missing something.

What it does:

- if CLUSTER is called with no arguments, cluster all indexes that have
indisclustered set (in the current database). There's no "ALL"
argument: that's just pollution IMHO.

- Gets a list of such indexes (checking ownership of each) and passes
them one by one to the standard cluster routine (modified a little so
it accepts OIDs of table and index, not names).

- Not much else... (no regression test nor documentation yet).

I don't know if I will be able to do the REINDEX all thing now that beta
is almost here.

Please review; I may be doing something stupid.

--
Alvaro Herrera (<alvherre[a]atentus.com>)
"Aprender sin pensar es inutil; pensar sin aprender, peligroso" (Confucio)