Washington (AP) - The nation's highest civilian honor, the Medal of Freedom, was presented by President Nixon Wednesday to William J. Hopkins, a native Kansan who retired after 40 years' service in the White House. Hopkins, born in 1910 in Netawaka, began government service as a junior clerk and stenographer in Chillicothe, Mo., in 1928. On his retirement he was executive clerk in the White House office, which handles documents and correspondence and serves as a clearing house for polic