harmonize password reuse in vacuumdb, clusterdb, and reindexdb

While looking at something unrelated, I noticed that the vacuumdb docs
mention the following:

vacuumdb might need to connect several times to the PostgreSQL server,
asking for a password each time.

IIUC this has been fixed since 83dec5a from 2015 (which was superceded by
ff402ae), so I think this note (originally added in e0a77f5 from 2002) can
now be removed.

I also found that neither clusterdb nor reindexdb uses the
allow_password_reuse parameter in connectDatabase(), and the reindexdb
documentation contains the same note about repeatedly asking for a
password (originally added in 85e9a5a from 2005). IMO we should allow
password reuse for all three programs, and we should remove the
aforementioned notes in the docs, too. This is what the attached patch
does.

Thoughts?

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com