Doctors treating a Canadian woman who was stranded for seven weeks in a remote area near the Nevada-Idaho border say she was within days of dying from lack of nutrition.      Dr. James Westberry at St. Luke's Magic Valley Medical Center in Twin Falls said Sunday that Rita Chretien is on a liquid diet and is very weak, but making progress.  Her son says his mother and  father, Albert Chretien, ran into problems on their drive to Las Vegas. His father remains missing after leaving his wife behind to get help.  Dr. Westberry says Rita Chretien ate snacks the couple had with them for the trip, rationing her supply.  She was found Friday by hunters who spotted her van mired in mud on a national forest road in Elko County.