cp: support '--' terminator

[mknos]

• '--' option terminator is ignored when executing an external command
• When debugging this I discovered the command list passed to system() strips off the '--' (technically Getopt::Long does)
• Explicit terminator can be added between options and filenames

  %perl cp -- -a ..   # before patch, on linux
  cp: missing destination file operand after '..'
  Try 'cp --help' for more information.

[briandfoy]

changes: stop option parsing after --