RETRO SIXTIES Go Mild. Or Wild. That's the main fashion message. But it's not the only one. • BY BARBARA PASH F or reasons only a psychiatrist could fathom, this spring the Sixties are back — better than the first time they were here. American designers have sifted through the decade that wouldn't die, sepa- rating the gloss from the dross and en- capsulating the entire period into two looks. Time travelers, take your pick — Mild or Wild. Mild & Wild Mild goes by various monikers — The Jacqueline Kennedy, the Jackie 0 or the Babe Paley look — which all translate into a chic, ladylike image. In suits, there are short boxy jackets and slim skirts; in dresses, crisp shifts 58 51YLE