By David Shepardson and Allison Lampert WASHINGTON (Reuters) – The U.S. aviation regulator on Friday extended the grounding of Boeing 737 MAX 9 airplanes indefinitely for new safety checks and announced it will tighten oversight of Boeing itself after a cabin panel broke off a new jet in mid-flight. As United Airlines and Alaska Airlines canceled flights through Tuesday, the Federal Aviation Administration (FAA) said it will require another round of inspections before it will consider putting the jets back in service. Under more stringent supervision, the regulator will audit the Boeing 737 MA…