bpo-34521: Fix FD transfer in multiprocessing on FreeBSD (GH-15422)

Fix file descriptors transfer in multiprocessing on FreeBSD: use
CMSG_SPACE() rather than CMSG_LEN(); see RFC 3542.

Misc/NEWS.d/next/Library/2019-08-23-14-47-09.bpo-34521.Y2BYu5.rst

+Fix file descriptors transfer in multiprocessing on FreeBSD: use
+``CMSG_SPACE()`` rather than ``CMSG_LEN()``; see :rfc:`3542`.

Modules/_multiprocessing/multiprocessing.c

@@ -167,7 +167,7 @@ multiprocessing_recvfd(PyObject *self, PyObject *args)
     cmsg = CMSG_FIRSTHDR(&msg);
     cmsg->cmsg_level = SOL_SOCKET;
     cmsg->cmsg_type = SCM_RIGHTS;
-    cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+    cmsg->cmsg_len = CMSG_SPACE(sizeof(int));
     msg.msg_controllen = cmsg->cmsg_len;
 
     Py_BEGIN_ALLOW_THREADS