DETROIT — The last time there was a white mayor of Detroit, the Vietnam War was just ending and the nation was getting used to a Supreme Court decision called Roe vs. Wade. But now this city, which is in the midst of a trial to determine whether it is eligible for bankruptcy protection, is set to elect its first white mayor since 1974, Mike Duggan, and by an overwhelming margin. Recent polls show Duggan up by a nearly 2-1 margin over his opponent, Wayne County Sheriff Benny Napoleon, in a city that is 82% African American.