ISTANBUL — Turkey’s President Recep Tayyip Erdogan declared victory after unofficial election returns Sunday showed him with enough votes to serve another term that carries new executive powers. “The nation has entrusted to me the responsibility of the presidency and the executive duty,” Erdogan said in televised remarks from Istanbul after a near-complete count carried by the state-run news agency gave him the majority needed to avoid a runoff. Recep Tayyip Erdogan The presidential election and a parliamentary election also held Sunday, both more than a year early, complete NATO-member Turkey’s transition from a parliamentary system of government to a strong presidential system.