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
a179232047 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.

src/bin/scripts/createuser.c

@@ -250,8 +250,8 @@ main(int argc, char *argv[])
 	if (login == 0)
 		login = TRI_YES;
 
-	conn = connectDatabase("postgres", host, port, username, prompt_password,
-						   progname, false);
+	conn = connectDatabase("postgres", host, port, username, NULL,
+						   prompt_password, progname, false);
 
 	initPQExpBuffer(&sql);