PBS programming is staid and unadventurous. Why? Consider an acclaimed 36-year-old one-man show about Mark Twain and why PBS declined to air it: “What could literary legend Mark Twain have said 130 years ago that would cause PBS programmers of two different regimes to reject an acclaimed performance of his wit and wisdom by one of America’s most respected actors? Well, about 35 minutes into the one-man show, after the first intermission, Twain/ Holbrook includes a passage ofHuckleberry Finn. In this five minutes of the program, “Twain” acts out the parts of young Huck, his drunk father Pap and old Jim, a slave…”