GARRISON KEILLOR at 80 is a show of music, stories, and stand-up on the theme of cheerfulness — happiness depends on circumstance but cheerfulness is a choice. The show includes Keillor’s sung sonnets (“Prayer,” “Longevity,” “Love Song”) and duets with Masse on songs by Bob Dylan, Paul Simon, Tom Waits, Greg Brown, Mark Knopfler, and Ann Reed, plus an extended medley of sung classic poems and jokes.   Keillor also does the News from Lake Wobegon, reflecting on his generation, the one that knew about outhouses, slaughtered chickens, hitchhiked, drove a straight-stick transmission, skated on outdoor rinks, and told jokes.  The program concludes with the audience singing a cappella an impromptu medley of familiar songs — “America,” “Swing Low, Sweet Chariot,” “Oh Susannah,” “In My Life,” “Going to the Chapel,” etc. “We are the last generation who knows all the words,” says Keillor. “When we’re gone, they’ll disappear.”