Nigerian President Muhammadu Buhari has secured a second term helming Africa’s largest economy and top oil producer, but the former general now faces a dizzying array of challenges including a divided population, moribund economy and a rejuvenated Islamist insurgency. Nigeria’s electoral commission early Wednesday confirmed Mr. Buhari had beaten his opponent, Atiku Abubakar by a margin of four million votes in a fractious election. The campaign was marred by delays, ballot manipulations and violence that left 39 people dead….