By Khaled Yacoub Oweis ISTANBUL (Reuters) - Syria's opposition resumed talks on Saturday aimed at closing their fractious ranks, as government forces launched a fierce onslaught on a rebel-held border town to try to gain the upper hand in the civil war. A failure of the opposition to unite could weaken the hand of Russia and the United States, co-sponsors of a proposed peace conference on the war, which has killed 80,000 and threatens to spill over borders and whip up wider sectarian violence. The U.S. ...