By Stephanie Nebehay GENEVA (Reuters) - Islamic State insurgents in Iraq have carried out mass executions, abducted women and girls as *** slaves, and used child soldiers in what may amount to systematic war crimes that demand prosecution, the United Nations said on Thursday. In a report based on 500 interviews with witnesses, also said Iraqi government air strikes on the Sunni Muslim militants had caused "significant civilian deaths" by hitting villages, a school and hospitals in violation of international law. ...