Issue #6559: fix the subprocess.Popen pass_fds implementation. Add a unittest.

Issue #6559: fix the subprocess.Popen pass_fds implementation. Add a unittest.
Issue #7213: Change the close_fds default on Windows to better match the new
default on POSIX.  True when possible (False if stdin/stdout/stderr are
supplied).

Update the documentation to reflect all of the above.