NEW DELHI — He was supposed to be a faceless accountant, but he has become a household name in India and perhaps the central actor in the nation’s battle against corruption. The slim, silver-haired Vinod Rai is the government’s chief auditor, a man appointed on the recommendation of the prime minister more than five years ago. Now he has turned into one of the government’s greatest scourges, a hero to many but a source of controversy to others.