Getting a second Heisman Trophy winner for its second season, which opens Sunday, may seem like icing on the cake, but the United States Football League needs more than Mike Rozier (below) and Herschel Walker (left) to ensure future birthdays. TV ratings and attendance must increase from '83's averages (6.0 and 25,000) to 7.0 and 35,000 or its flame may flicker. Turn the pages for scouting reports and a profile of a gutsy Gambler