* Mass executions, abductions by IS deemed war crimes * UN rights chief de**unces "staggering array" of abuses * Iraqi air strikes cause "significant civilian deaths" (adds detail, quotes, byline) By Stephanie Nebehay GENEVA, Oct 2 (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. ...