By GINIA BELLAFANTE

He mastered the master's vibrato; now he's aiming for the bravado. HARRY CONNICK JR. tries hard to be like his idol, Frank Sinatra, but he's never measured up in the bad-boy department. Harry has always seemed too refined, too incapable of referring to women as "broads." But no need to sanctify the 25-year-old showman just yet. Last week Harry spent the night in a New York City jail. Reason: the flannel-shirt-clad crooner forgot to license a new gift, an unloaded 9-mm pistol (what ever happened to Old Spice?), before heading off to the airport with it. Scheduled to fly to New Orleans, Harry was detained and arraigned instead. His prison request? Some ice cream. Ol' Blue Eyes would have opted for Jack Daniel's.