bpo-38417: Add umask support to subprocess (GH-16726)

On POSIX systems, allow the umask to be set in the child process before we exec.
diff --git a/Lib/multiprocessing/util.py b/Lib/multiprocessing/util.py
index 207a2f7..3e640b9 100644
--- a/Lib/multiprocessing/util.py
+++ b/Lib/multiprocessing/util.py
@@ -429,7 +429,7 @@
         return _posixsubprocess.fork_exec(
             args, [os.fsencode(path)], True, passfds, None, None,
             -1, -1, -1, -1, -1, -1, errpipe_read, errpipe_write,
-            False, False, None, None, None, None)
+            False, False, None, None, None, -1, None)
     finally:
         os.close(errpipe_read)
         os.close(errpipe_write)