Luke Holwerda"Spinal Tap" metal and "A Mighty Wind" folk each got their due Monday night.Before the Unwigged & Unplugged show even started last night, the official Spinal Tap moment had already happened. When the announcer came onstage at Dodge Theatre to do the customary plugging of upcoming shows, he looked over the half-full house and noted that the guys backstage had asked him if they'd advertised this show. It's too bad that more people didn't get to see Christopher Guest, Michael McKea