Being a presidential also-ran has its privileges. John Kerry, who went backstage after U2's recent show in Boston, walked into Cambridge Music the other day with a sweet guitar given to him by Bono. He needed to get the custom ax appraised because pols need to report the value of all gifts. The Gretsch ''Bono Irish Falcon" lists for $4,500. Before leaving, Kerry paid $600 for a Coupe tube amplifier and bought a book, ''Essential Licks for Guitars," a DVD, ''Beginning Rock Guitar: Lead and Rhythm," assorted picks, and three sets of strings. . . .
