WASHINGTON — U.S. unemployment fell to a seven-year low of 5.1 percent last month, but hiring slowed — a mixed bag of news that offers few clues to whether the Federal Reserve will raise rock-bottom interest rates later this month. The Labor Department report, issued Friday, was closely watched because it will be the last snapshot of the job market before the Fed meets in two weeks. And overall, it painted a picture of an economy growing at a modest but steady pace seven years after the Great Recession.