TVNewser has learned legendary CBS newsman Walter Cronkite, 92, who once held the title of "Most Trusted Man in America," is said to be gravely ill, according to multiple CBS News sources. The network began updating his obituary more than a week ago, a source adds. CBS News executive Linda Mason, designated to speak on Cronkite's behalf, had no comment. The avuncular Cronkite anchored "CBS Evening News" for 19 years, until 1981, when he was forced to retire. Dan Rather was named his successor. Cronkite maintained an office at CBS, where he was a special correspondent. A native of St....