No one is inside that boarded-up mansion on Manhattan's ritzy 5th Avenue, right? Wrong. Its secret inhabitants include a philosophizing hobo and some GI families who are unable to find apartments during the post-World War II housing crunch. They set up residence in the posh brownstone while the owner, the second richest man in the world, winters in Virginia. Except the owner isn't in Virginia. He's there in disguise and discovering a lo