Tommy Prine is sad about something. But it’s not an indulgent, wallowing, “poor me” kind of sadness. What saddens him is just a fact of life, and he knows it has to be that way. He can’t change it, so all he can do is acknowledge it, live with it, accept it for what it is. Oh, and one more thing: He can write a song. The tune is “Ships in the Harbor,” and it became the 26-year-old Prine’s debut single when it was released in September, prelude to an album set for release early in the new year. Part…