By DOUGLAS FEIDENDAILY NEWS STAFF WRITER Postal Inspector General Karla Corcoran is lifted by 500 postal workers at conference last year in Washington. WASHINGTON — They bark like a pack of dogs, quack like a flock of ducks and hiss like a nest of vipers. They wrap each other from head to toe in toilet paper and aluminium foil and pipe cleaners. They build sandcastles and gingerbread houses and practice picking up oranges while blindfolded. These are the professional auditors and investigators who police the United States Postal Service. The mission of the USPS Office of Inspector General is...