By David Shepardson WASHINGTON (Reuters) -Tesla is recalling just over two million vehicles in the United States fitted with its Autopilot advanced driver-assistance system to install new safeguards, after a safety regulator said the system posed safety concerns. The National Highway Traffic Safety Administration (NHTSA) has been investigating the electric automaker led by billionaire Elon Musk for more than two years over whether Tesla vehicles adequately ensure that drivers pay attention when using Autopilot. The largest ever Tesla recall appears to cover nearly all of its vehicles on U.S. r…