issue7213: Open the pipes used by subprocesses with the FD_CLOEXEC flag from

the C code, using pipe2() when available.  Adds unittests for close_fds and
cloexec behaviors.

Lib/test/subprocessdata/fd_status.py

@@ -0,0 +1,24 @@
+"""When called as a script, print a comma-separated list of the open
+file descriptors on stdout."""
+
+import errno
+import os
+import fcntl
+
+try:
+    _MAXFD = os.sysconf("SC_OPEN_MAX")
+except:
+    _MAXFD = 256
+
+def isopen(fd):
+    """Return True if the fd is open, and False otherwise"""
+    try:
+        fcntl.fcntl(fd, fcntl.F_GETFD, 0)
+    except IOError as e:
+        if e.errno == errno.EBADF:
+            return False
+        raise
+    return True
+
+if __name__ == "__main__":
+    print(','.join(str(fd) for fd in range(0, _MAXFD) if isopen(fd)))