NetworkPkg: Refine the code of shell app under networkPkg.
    
    refine the code of ifconfig6 and ping6 application.
    
    Cc: Fu Siyuan <siyuan.fu@intel.com>
    Cc: Wu Jiaxin <jiaxin.wu@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
    Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
    Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>