VIENNA — Mohamed Mahmoud, a 26-year-old Austrian, wears a traditional white tunic and a light-brown hat and keeps his hair long, as is the tradition in the Pashtun tribal areas of Pakistan and Afghanistan. Only his camouflage vest and dark-blue sneakers place him in the modern world here in Europe, which he re-entered Sept. 12 after serving a four-year prison sentence for joining and actively supporting Al Qaeda and its affiliates.