NEW YORK/DETROIT, Aug 18 (Reuters) – Football players at Northwestern University may have lost a bid to unionize on Monday, but a broader legal challenge has targeted the practice of excluding college athletes from sharing in the multibillion dollar bonanza they help generate. Alleging the current system of providing scholarships, but no pay, to the country’s best athletes violates U.S. antitrust law, a group of football and men’s basketball players is suing the National Collegiate Athletic Association (NCAA).