MAINE State Police detectives have arrested a 55-year-old man in the grisly 1980 slaying of Rita St. Peter, 20. On July 5, 1980, cops say the attractive young woman had been drinking at a bar in Madison and staying with friends in nearby Anson, working at a drive-in restaurant. She was last seen walking across a bridge in the early morning hours until her body was found dumped later that day. Cops said at the time they had suspects in the slaying but couldn't seal the deal and the case went cold. Although detectives would not 