UNITED NATIONS (Reuters) - Countries that act unilaterally on the world stage undermine the authority of the United Nations and weaken the broad consensus needed to confront global problems, Pope Benedict said on Friday. In a major speech to the U.N. General Assembly, the pope also said the international community sometimes had the duty to intervene when a country could not protect its own people from "grave and sustained violations of human rights." The pope, who arrived from Washington on the second leg of a six-day U.S. trip, was only the third pontiff in history to address the General Assembly....