ShellPkg: Fix command-line parsing to start with Argv[0] when comparing passed-in options

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chris Phillips <chrisp@hp.com>
Reviewed-By: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15432 6f19259b-4bc3-4df7-8a09-765794883524