Donald J. Trump and Hillary Clinton said this week that we should think about shutting down parts of the Internet to stop terrorist groups from inspiring and recruiting followers in distant lands. Mr. Trump even suggested an expert who’d be perfect for the job: “We have to go see Bill Gates and a lot of different people that really understand what’s happening, and we have to talk to them — maybe, in certain areas, closing that Internet up in some way,” he said on Monday in South Carolina.