Airlines and Airports across the country are scrambling to meet new Federal Aviation Administration security rules the wake of yesterday's terrorist attacks in New York and Washington. Twin Cities-based Northwest Airlines has received government clearance to begin bringing back SOME international flights which were diverted to cities outside of the United States yesterday. And Minneapolis St. Paul Airport officials say the airport here will be ready to open UNDER HEIGHTENED SECURITY when the FAA permits the resumption of regular scheduled service. But NO ONE knows WHEN that will be.