PR#5568: add O_CLOEXEC flag to Unix.openfile, so that the returned
file descriptor is created in close-on-exec mode.
(Reflecting commit r13961 on version/4.01)


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13962 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02