British director Ken Loach has for 50 years been making lauded dramas rich in social realism that give dignity to the dispossessed. But his “I, Daniel Blake,” which won the Cannes Film Festival’s Palme d’Or, has powerfully struck a nerve, tapping into the same working-class frustrations that are upending politics in Europe and the U.S. The film, which is now in limited release, has emerged as a kind of rallying cry, part film and part movement.