CBS Defends Report on Bush Guard Memos WASHINGTON - CBS News mounted an aggressive defense Friday of its report about President Bush's service in the Air National Guard, with anchor Dan Rather saying broadcast memos questioned by forensic experts came from "what we consider to be solid sources."   On Friday's "CBS Evening News," Rather said that "no definitive evidence" has emerged to prove the documents are forgeries.   "If any definitive evidence comes up, we will report it," Rather said...