by Claudine Zap Brace yourselves: Prep is back . Prepare to put on that frayed Polo shirt, don the duck boots and grab your L.L. Bean tote. But instead of " The Official Preppy Handbook " to guide you, there's the new, updated version: " True Prep: It's a Whole New Old World ." The author, Lisa Birnbach , has waited 30 years since her last publication on prepdom to teach the latest generation about the ways of the preppie. This time around,