Automobile sales fell sharply in the United States last month, giving another sign that the problems in the financial sector are working their way through to the general economy.   All the major auto manufacturers saw sales fall in September compared with the same month a year earlier, with Nissan suffering a 37 percent decline.    Sales at Ford were down 34 percent over last September, while Chrysler was down 33 percent, Toyota 32 percent and Honda 24 percent.