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>

NetworkPkg/Application/Ping6/Ping6.c

@@ -30,6 +30,11 @@
 
 #include "Ping6.h"
 
+//
+// String token ID of Ping6 command help message text.
+//
+GLOBAL_REMOVE_IF_UNREFERENCED EFI_STRING_ID mStringPing6HelpToken = STRING_TOKEN (STR_PING6_HELP);
+
 SHELL_PARAM_ITEM    Ping6ParamList[] = {
   {
     L"-l",