By John Ruwitch and Ju-min Park CHANGBAI, China/SEOUL (Reuters) - Chinese currency and U.S. dollars are being used more widely than ever in North Korea instead of the country's own money, a stark illustration of the extent to which the leadership under Kim Jong-un has lost control over the economy. The use of dollars and Chinese yuan, or renminbi, has accelerated since a disastrous revaluation of the North Korean won in 2009 wiped out the savings of millions of people, said experts on the country, defectors and Chinese border traders. ...