'Deep Throat' Unmasks Himself as Ex-No. 2 Official at F.B.I.
W Mark Felt, No. 2 man at FBI in 1972, acknowledges in Vanity Fair magazine that he was 'Deep Throat,' mysterious figure who helped Washington Post unravel Watergate scandal and presidency of Richard M Nixon; was long a prime suspect to Nixon himself; Post confirms that Felt was official who encouraged its reporters, Bob Woodward and Carl Bernstein, to follow trail from break-in at Democratic National Committee headquarters in Watergate complex to highest levels of Nixon administration; Woodward and Bernstein initially decline to confirm Vanity Fair article, believing they promised Felt unconditional confidentiality until his death; Post, which has tightly guarded secret, suddenly finds itself scrambling to deal with monthly magazine's scoop of final footnote to biggest story in its history; photos (L)