Layovers have long been the bane of jet-setters’ existence, but Lufthansa has finally figured out how to solve the insufferable problem. The German airline is now offering first-class passengers and private-jet guests in Frankfurt and Munich the opportunity to rent a Porsche 911 or Panamera for an afternoon of sightseeing prior to departure. Available on the day of travel for all Lufthansa, Swiss, and Austrian Airlines passengers holding first-class tickets, the Porsche First Class Excitement service can turn …