Add a  --ocamlc-version option to lurettetop and lutin.

The idea is to put all the .exp files into a `lurettetop/lutin --ocamlc-version`
dir so that I have an .exp file for each ocaml version.

If the `lurettetop --ocaml-version`/*.exp does not exist, I generate it.

Also, don't non-reg test lucky dirs anymore (because they are boring).

Aldo, don't rely on the HOSTTYPE env var, but use Sys.os_type instead.