These last weeks of David Letterman’s shows – and yes, we’re down to that, because the final Late Show with David Letterman is now less than a month away – are increasingly rewarding to watch, with guests saying their proper farewells, after which baseball-card-style graphics display a photo of that guest’s first TV appearance with Dave. Vintage clips, especially of classic field pieces with Letterman and his crew out on the streets, also are doled out almost nightl