BY DAY-A PART-TIME SECRETARYBY NIGHT-A FULL-TIME PLAYGIRL"How long ago was that?" he asked. In the glow of the match her face seemed almost too angelic to be real. An angelic call-girl, he thought."About a year ago," she told him. "But it seems longer.""How'd it all begin, Kathy?" he asked suddenly.She glanced at him quickly, then away. "They always ask that," she said softly. "Always.""I'm sorry," Steve said, meaning it.She pursed her full lips and blew smoke out slowly, gazing toward the river."There was a man, of course," she said. "There always is."