By Joseph Menn WASHINGTON (Reuters) - Even as the U.S. government confronts rival powers over widespread Internet espionage, it has become the biggest buyer in a burgeoning gray market where hackers and security firms sell tools for breaking into computers. The strategy is spurring concern in the technology industry and intelligence community that Washington is in effect encouraging hacking and failing to disclose to software companies and customers the vulnerabilities exploited by the purchased hacks. That's because U.S. ...