POSIX systems with a C extension module. This is required in order for
the subprocess module to be made thread safe.
The pure python implementation is retained so that it can continue to be
used if for some reason the _posixsubprocess extension module is not
available.
The unittest executes tests on both code paths to guarantee compatibility.
* Moves PyLong_FromPid and PyLong_AsPid from posixmodule.c into longobject.h.
Code reviewed by jeffrey.yasskin at http://codereview.appspot.com/223077/show