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/clusterdb.c

@@ -203,7 +203,7 @@ cluster_one_database(const char *dbname, bool verbose, const char *table,
 		appendPQExpBuffer(&sql, " %s", table);
 	appendPQExpBufferChar(&sql, ';');
 
-	conn = connectDatabase(dbname, host, port, username, prompt_password,
+	conn = connectDatabase(dbname, host, port, username, NULL, prompt_password,
 						   progname, false);
 	if (!executeMaintenanceCommand(conn, sql.data, echo))
 	{