add code for switching unsafe_string in Compmisc.initial_env

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/safe-string@14681 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
split Env.initial, and propagate real initial_env

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/safe-string@14682 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02