ShellPkg: Refactor string manipulation in cp command

This patch replaces StrCpy with StrnCpy or refactors out the usage of StrCpy through some other means.
This patch replaces StrCat with StrnCat or refactors out the usage of StrCat through some other means.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@15771 6f19259b-4bc3-4df7-8a09-765794883524