MAIDUGURI, Nigeria — Suspected Islamist fighters killed at least 20 Nigerian soldiers, including a commanding officer, after attacking an army base in a remote town in northeastern Borno state, security sources and residents said Sunday. Boko Haram and Islamic State West Africa Province (ISWAP) fighters have mainly operated in Borno, targeting security forces and civilians, in the process killing and displacing tens of thousands of people. The latest assault happened Friday, when ISWAP members arrived on gun trucks and attacked the army’s 149 Battalion in Malam-Fatori town, gateway to the border with Niger, two soldiers and residents said. One of