I attach a little patch to make CLUSTER set and reset the indisclustered

I attach a little patch to make CLUSTER set and reset the indisclustered
bit on the indexes.

I also attach clusterdb and clusterdb.sgml; both of them are blatant
rips of vacuumdb and vacuumdb.sgml, but get the job done.  Please review
them, as I'm probably making a lot of mistakes with SGML and I can't
compile it here.

vacuumdb itself is not very comfortable to use when the databases have
passwords, because it has to connect once for each table (I can probably
make it connect only once for each database; should I?).  Because of
this I added a mention of PGPASSWORDFILE in the documentation, but I
don't know if that is the correct place for that.

Alvaro Herrera