MAIDUGURI, Nigeria( Reuters)-At least four police officers were killed in Nigeria's northeast Borno state after a gun battle with suspected Islamist insurgents, a police spokesperson said on Saturday. Borno has been the heartland of an insurgency by Boko Haram and splinter outfit Islamist State West Africa Province( ISWAP) for more than a decade in the northeast, where they continue to carry out deadly attacks against civilian and security targets. Borno state police spokesperson Nahum Daso Kenn