GENEVA, Sept 1 (Reuters) - Islamic State forces are committing atrocities amounting to crimes against humanity against mi**rities while Iraqi government forces have executed detainees and shelled civilian areas in acts that may constitute war crimes, the United Nations said on Monday. U.N. Deputy High Commissioner for Human Rights Flavia Pansieri, opening an emergency session of the U.N. Human Rights Council in Geneva, said that the conflict is having a grave impact on Iraqi civilians, particularly women and children. ...