BEIJING (Reuters) - Vice President Dick Cheney started a three-day visit to China Tuesday aimed at keeping a lid on tension between China and Taiwan and jump-starting talks to curb North Korea's nuclear ambitions. In his first closed-door meeting after arriving from Tokyo, Cheney reassured Chinese leaders that the United States did not support Taiwan independence and would oppose unilateral steps by either side, according to China's official Xinhua news agency. But Washington is increasingly concerned by China's build-up of ballistic missiles capable of striking Taiwan, and Cheney was expected to raise the issue when talks resume on Wednesday. China,...