PROBABLY THE LEAST SEXY thing to happen to women's legs in the past 30 years is pantyhose. But now Hanes and L'eggs, twin sisters of U.S. hosiery (and daughters of Sara Lee, Corp., better known for a different kind of cheesecake) plan to put some sizzle into selling pantyhose. L'eggs has named Jamie Lee Curtis, known for fitness and curves, its model and "voice." Hanes will get a leg up by appointing sensual soul singer Tina Turner to the same position. They'll show off their gams (Curtis's are insured for $1 million by Lloyd's of London) in TV and print ads and store displays. Why did Hanes like Turner, at 56 an outspoken, rebellious, free spirit more than a corporate type? "She's a woman with a beautiful soul," says Cathy Volker, president of Sara Lee's hosiery division, "who also happens to have the best legs in the world."