Robert Mugabe has been assured of becoming Zimbabwe's first executive president and realizing his goal of creating a one-party socialist state. As prime minister, he was the only candidate nominated for the new office after he and his longtime political foe, Joshua Nkomo, agreed Wednesday to merge the African country's two main political parties. As executive president, Mugabe, 63, will combine the posts of chief of state, prime minister and armed forces commander. His first term, beginning next Thursday, will be for six years. There is no limit to the number of terms he can serve.