At Hours before daybreak, Frank Acosta treads silently down apartment corridors and slips beneath townhouse stoops. He's got to get glass-bottled milk to his customers' doors in time for their kids to splash it on breakfast cereal. Manhattan, the milkman is back. And he's headed for Brooklyn, too. In the next month or so, Acosta and business partner Matt Marone will expand their company, Manhattan Milk, into brownstone neighborhoods like Brooklyn Heights, Carroll Gardens and Park Slope. Even before crossing the East River, they're getting an earful of milkman jokes.