vacuumdb: don't prompt for passwords over and over

Having the script prompt for passwords over and over was a preexisting
problem when it processed multiple databases or when it processed
multiple analyze stages, but the parallel mode introduced in commit
a1792320 made it worse.

Fix the annoyance by keeping a copy of the password used by the first
connection that requires one.  Since users can (currently) only have a
single password, there's no need for more complex arrangements (such as
remembering one password per database).

Per bug #13741 reported by Eric Brown.  Patch authored and
cross-reviewed by Haribabu Kommi and Michael Paquier, slightly tweaked
by Álvaro Herrera.

Discussion: http://www.postgresql.org/message-id/20151027193919.931.54948@wrigleys.postgresql.org
Backpatch to 9.5, where parallel vacuumdb was introduced.