CAIRO — The 20-year-old law student says he has had enough of fruitless protests in support of Egypt’s deposed Islamist president, two years of a losing struggle with police. Now he wants to join the extremists of the Islamic State group who are battling the army in the Sinai Peninsula. He and other youths are growing increasingly open in their calls for violence and a move toward extremism, frustrated by the police crackdown since the military ousted President Mohammed Morsi in 2013.