A few nights after he resigned his post as secretary of state two years ago, Colin L. Powell answered a ring at his front door. Standing outside was Prince Bandar, then Saudi Arabia?s ambassador to the United States, with a 1995 Jaguar. Mr. Powell?s wife, Alma, had once mentioned that she missed the ?95 Jag that she and her husband had traded in a while back. Prince Bandar had filed that information away, and presented the Powells that night with an identical, 10-year-old model. The Powells kept the car ? a gift that the State Department said was legal ? but recently traded it away.