FORMER First Daughter Chelsea Clinton and her longtime boyfriend Ian Klaus have called it quits. "They are no longer together," a source close to Clinton confirmed. And Klaus isn't wasting any time getting back to the dating business. Early last week, the tousled-haired history buff - having recently returned from a lengthy stint in Iraq doing humanitarian work - was seen romancing a "curly blond-haired girl, looking very much together," at the new downtown in spot, Employees Only on Hudson Street, another reliable source told PAGE SIX. -snip- Having now grown out of her gangly appearance during Bill Clinton's years...