The old saying that there are no second chances in the theater is belied by tenor Stuart Neill, who’s getting his fourth wind. After early successes, including recordings with Colin Davis and Michael Tilson Thomas, he had a five-year exile from opera, retooling his voice and singing Christmas carols in suburban Philadelphia. Now, having saved La Scala’s opening night (and its worldwide simulcast), Neill is finally getting re-engaged. He tells David Patrick Stearns what it took to get there.