by Gore Vidal. Random House, $6.95. Myra Breckinridge is back, disputing with her male alter ego for possession of what is, by now, a peculiarly indeterminate carcass. Mr. Vidal uses her/him to distribute satirical whacks at thirty years of American follies, which perhaps requires rather too long a reach for any whack to hit solidly home. Certainly this sequel, despite some wonderfully amusing sections, never quite equals the spirited outrageousness of its predecessor.