One by one, starting a few weeks ago, 40 militant Islamist Web sites got knocked off the Internet. Gone were some of the world’s most active jihadi sites, with forums full of extremist chatter. This disappearance mystified American counterterrorism officials. They hadn’t shut them down, they knew, so who had? Happily claiming credit for the jihadi blackout is a Christian-Lebanese engineer named Joseph G. Shahda, who is waging a private, and passionate, war on terrorism from his home near Boston.