BEIRUT — A Lebanese man with a shotgun took up to 10 employees and customers hostage at a Beirut bank Thursday and threatened to set himself on fire with gasoline unless he was allowed to withdraw some of his trapped savings to pay his father’s medical bills. Soldiers and police converged on the area and sought to negotiate an end to the standoff. The hostage drama in Beirut’s bustling Hamra district was the latest painful episode in Lebanon’s economic free-fall, now in its third year.