Voluntary Flu Vaccine Rationing Urged By LAURAN NEERGAARD     WASHINGTON (AP) -- A top federal health official, lamenting "a very fragile vaccine production system," urged healthy people Wednesday to defer getting their influenza shots so medication will be available for those most at risk.    "We really need a long-term solution so we don't end up in this year-to-year situation where we don't have a reliable supply," Dr...