What Fletch was to plaid-checked water-cooler wits in the ’80s, what National Lampoon’s Van Wilder was to college-bound douches at the dawn of Dubya, that’s what 2003’s Old School is to Gen-X frat rats — a secret-handshake movie. A shaggy, intermittently hilarious wish-fulfillment nightmare about sorta dissatisfied, sorta middle-aged dudesters…