INT B3 - ZIPKEY - GET NAME OF PRIMARY CITY FOR A ZIPCODE REGION
        AH = 7Eh
        DX = zipcode region (0-999)
        ES:DI -> buffer for name
Return: CF set on error
            AL = FFh region does not exist
        CF clear if successful
            AL = ZIPKEY state code
            ES:DI points one byte beyond end of name