NIGERIAN President Muhammadu Buhari has appointed a new military high command, after years of mounting criticism over spreading violence by Islamist insurgents and armed gangs. "President Buhari thanks the outgoing Service Chiefs for what he calls their 'overwhelming achievements in our efforts at bringing enduring peace to our dear country'," the presidency spokesman said. He told Reuters later that some of the chiefs had resigned while others retired. Leo Irabor was named to the powerful Chief of Defence Staff post, which oversees the main military branches, the presidency spokesman said, while I. Attahiru, A.Z. Gambo and I.O. Amao would command…