Possible Flu Outbreak Threatens U.S. By LAURAN NEERGAARD    WASHINGTON (AP) -- The United States may have to close schools, restrict travel and ration scarce medications if a powerful new flu strain spurs a worldwide outbreak, according to federal plans for the next pandemic, obtained Wednesday by The Associated Press.    It will take months to brew a vaccine that works against the kind of super-flu that causes a pandemic, although government preparations include research to speed that production...