PR#5982: save and restore the value of errno in signal handlers


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13667 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#5982: save and restore the value of errno in caml_leave_blocking_section


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13668 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02