Spirit Airlines is ending service in a host of U.S. cities this fall as it fights for survival. Its rivals, meanwhile, are circling its customers, and one is preparing for the possibility the budget travel icon shuts down altogether.United Airlines on Thursday put on sale a slew of new flights in cities where Spirit operates, like its home base of Fort Lauderdale, Florida, and Orlando, Florida, as well as Las Vegas, Houston and Chicago."If Spirit suddenly goes out of business it will be incredibly disruptiv ...