BENGHAZI, Libya — Witnesses and the authorities have called Ahmed Abu Khattala one of the ringleaders of the Sept. 11 attack on the American diplomatic mission here. But just days after President Obama reasserted his vow to bring those responsible to justice, Mr. Abu Khattala spent two leisurely hours on Thursday evening at a crowded luxury hotel, sipping a strawberry frappe on a patio and scoffing at the threats coming from the American and Libyan governments.