A FIRST PLANELOAD of returning political exiles arrived in South Africa Thursday to cheers from hundreds of friends and relatives. But the African National Congress, which many of the exiles belong to, warned the almost 100 returning exiles and their families to expect resettlement problems . . . RETIRED DEPUTY police chief Jack Warren, whose arrest of Martin Luther King Jr. in 1963 led to King's famous "Letter from Birmingham Jail," has died at age 73 in Birmingham, Ala.