By Kareem Raheem BAGHDAD (Reuters) - At least 79 people were killed in a series of car bombings and suicide attacks targeting Shi'ite Muslims across Iraq on Monday, police and medics said, extending the worst sectarian violence since U.S. troops withdrew in December 2011. The attacks increased the number killed in sectarian clashes in the past week to more than 200. Tensions between Shi'ites, who now lead Iraq, and minority Sunni Muslims have reached a point where some fear a return to all-out civil conflict. No group claimed responsibility for the bombings. ...