GLOBAL MARKETS-Stocks struggle, dollar down; U.S. manufacturing slips

* Contraction in U.S. manufacturing is first in six months * Dollar falls below 100 yen, lowest since early May * Stocks fall on weak U.S. data in volatile trading NEW YORK, June 3 (Reuters) - Stocks and the U.S. dollar struggled on Monday in a seesaw session after data showed U.S. manufacturing activity contracted for the first time in six months, raising concerns about the health of the U.S. recovery. The Institute for Supply Management reported that its gauge for new orders