Add the -P command line option and the PYTHONSAFEPATH environment
variable to not prepend a potentially unsafe path to sys.path.

* Add sys.flags.safe_path flag.
* Add PyConfig.safe_path member.
* Programs/_bootstrap_python.c uses config.safe_path=0.
* Update subprocess._optim_args_from_interpreter_flags() to handle
  the -P command line option.
* Modules/getpath.py sets safe_path to 1 if a "._pth" file is
  present.