procutil: introduce pseudo file object that just raises EBADF
This should be safer than closing underlying fd as the fd may be reused.
On Python 2, closed sys.stdin could be redirected to a random file having
fd=0, but we'd be better not copying this behavior.
Only readinto() and write() are implemented according to the following ABC
table. fileno() is not implemented since fd=0/1/2 may be assigned later
to other files.