ShellPkg: Enhance ping6 to select the interface automatically

ShellPkg: Enhance ping6 to select the interface automatically

v2:
* Refine the code to make it more readable.

This patch is used to support no source IP specified case while
multiple NICs existed in the platform. The command will select the
first both connected and configured interface automatically.
Note: Source address is always required when pinging a
link-local address.

Cc: Bhupesh Sharma <bhupesh.sharma@nxp.com>
Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>