PR 6439 part 2: use sysconf instead of the deprecated getpagesize function

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14925 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
improve finaliser documentation (PR 6355)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14927 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02