By David Shepardson WASHINGTON (Reuters) -The U.S. Federal Aviation Administration (FAA) has recommended that airlines operating Boeing 737-900ER jets inspect door plugs to ensure that they are properly secured after some operators reported unspecified issues with bolts upon inspections. Regulators have stepped up scrutiny on Boeing after a Jan. 5 mid-air panel blowout on an eight-week-old Alaska Airlines MAX 9 jet that left a gaping hole in the aircraft. Boeing has sought to contain the damage, appointing an independent advisor to examine quality control in its manufacturing processes while n…