“ Upon a proper showing a Court of Appeals has a duty to displace a District Court's certification. Moreover, a Court of Appeals must, under Johnson v. Zerbst, 304 U.S. 458, 58 S.Ct. 1019, 82 L.Ed. 1461, afford one who challenges that certification the aid of counsel unless he insists on being his own. Finally, either the defendant or his assigned counsel must be enabled to show that the grounds for seeking an appeal from the judgment of conviction are not frivolous and do not justify the finding that the appeal is not sought in good faith. ”