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/createlang.c

@@ -140,8 +140,8 @@ main(int argc, char *argv[])
 		printQueryOpt popt;
 		static const bool translate_columns[] = {false, true};
 
-		conn = connectDatabase(dbname, host, port, username, prompt_password,
-							   progname, false);
+		conn = connectDatabase(dbname, host, port, username, NULL,
+							   prompt_password, progname, false);
 
 		printfPQExpBuffer(&sql, "SELECT lanname as \"%s\", "
 				"(CASE WHEN lanpltrusted THEN '%s' ELSE '%s' END) as \"%s\" "
@@ -180,8 +180,8 @@ main(int argc, char *argv[])
 		if (*p >= 'A' && *p <= 'Z')
 			*p += ('a' - 'A');
 
-	conn = connectDatabase(dbname, host, port, username, prompt_password,
-						   progname, false);
+	conn = connectDatabase(dbname, host, port, username, NULL,
+						   prompt_password, progname, false);
 
 	/*
 	 * Make sure the language isn't already installed