ShellPkg: Enhance ping to select the interface automatically

ShellPkg: Enhance ping to select the interface automatically

v2:
* A. Refine the code to make it more readable.
* B. Add hint message for link local address case.

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: David Van Arnem <dvanarnem@cmlab.biz>
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>