NEW YORK - Jean Shepherd ha a gentleman's paunch and a lopsided mustache that looks like a horseshoe with one leg stuck in the salt. For 10 or 15 years he has been American youth's favorite humorist, story-teller and accidental guru, devotion to him being for many a genuine rite of passage. There is a familiar scenario to it: the 18 year-old suburban couple in dad's car with a pizza and a bottle of Beaujolais, parked on some scenic spot listening to Shepherd's nightly syndicated radio show and feeling in-touch, aware - well, hip - at least more so than those creeps down at MacDonald's. . .