Gunmen have killed 17 people including five policemen in separate attacks in northwest Nigeria’s Katsina state in the latest wave of violence in the region, police and a local official said Thursday. Northwest and central Nigeria are a hub of criminal gangs known locally as bandits who raid villages, killing or abducting residents after looting and burning homes. The criminals, who maintain camps in a vast forest straddling Zamfara, Katsina, Kaduna and Niger states, were declared terrorists by Nigeria’s government in January. Late on Wednesday, a group of around 300 bandits on motorcycles atta…