[CP-7394] Ensure old --sign and --addcert commands continue to function

Use 'continue' to ensure argptr doesn't overrun the length of sys.argv

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
[CP-7394] Document --signcmd argument

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>