* Replaces the internals of the subprocess module from fork through exec on

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

Objects/abstract.c

@@ -2722,3 +2722,63 @@ PyIter_Next(PyObject *iter)
 		PyErr_Clear();
 	return result;
 }
+
+
+/*
+ * Flatten a sequence of bytes() objects into a C array of
+ * NULL terminated string pointers with a NULL char* terminating the array.
+ * (ie: an argv or env list)
+ *
+ * Memory allocated for the returned list is allocated using malloc() and MUST
+ * be freed by the caller using a free() loop or _Py_FreeCharPArray().
+ */
+char *const *
+_PySequence_BytesToCharpArray(PyObject* self)
+{
+	char **array;
+	Py_ssize_t i, argc;
+
+	argc = PySequence_Size(self);
+	if (argc == -1)
+		return NULL;
+
+	array = malloc((argc + 1) * sizeof(char *));
+	if (array == NULL) {
+		PyErr_NoMemory();
+		return NULL;
+	}
+	for (i = 0; i < argc; ++i) {
+		char *data;
+		PyObject *item = PySequence_GetItem(self, i);
+		data = PyBytes_AsString(item);
+		if (data == NULL) {
+			/* NULL terminate before freeing. */
+			array[i] = NULL;
+			goto fail;
+		}
+		array[i] = strdup(data);
+		if (!array[i]) {
+			PyErr_NoMemory();
+			goto fail;
+		}
+	}
+	array[argc] = NULL;
+
+	return array;
+
+fail:
+	_Py_FreeCharPArray(array);
+	return NULL;
+}
+
+
+/* Free's a NULL terminated char** array of C strings. */
+void
+_Py_FreeCharPArray(char *const array[])
+{
+	Py_ssize_t i;
+	for (i = 0; array[i] != NULL; ++i) {
+		free(array[i]);
+	}
+	free((void*)array);
+}