evidence: Jane Fonda and Tom Hayden, America's most famous antiwar couple, are making new overtures to this country's political mainstream and to its Vietnam veterans. Ms. Fonda, who took a controversial trip to Hanoi during the Vietnam war, goes on ABC-TV tonight to apologize to Americans who served in that conflict. [...] Fonda's conciliatory statements on Vietnam are drawing national attention, but are also raising questions about her motives. [...] Interviewed by Barbara Walters for the show ``20/20,'' Fonda talks about her 1972 trip, when she climbed onto a North Vietnamese antiaircraft gun for photographs, and made a propaganda broadcast from Hanoi.