GLOBAL MARKETS-Shares end flat, closing out strong week

* Equities breathe sigh of relief after Scotland vote * World stocks close out fifth week of gains in six * Alibaba up 38 percent in trading debut on heavy volume (Updates to close of U.S. trading) By Ryan Vlastelica NEW YORK, Sept 19 (Reuters) - Stock markets around the world closed flat on Friday as early gains failed to hold through the session, though the upward trend in equities continued and Alibaba soared in its historic U.S. trading debut. Alibaba Group, one of the