OPAM requires that the toplevel init checks `OCAML_TOPLEVEL_PATH`

The default `ocamlinit` installed by OPAM includes some logic to
add `OCAML_TOPLEVEL_PATH` to the search path.  This lets a system
installation of OCaml work with the OPAM-managed findlib packages.

This patch adds the check to the iocaml init, and also activates
`camlp4o` and `thread` (both part of the default recommended
ocamlinit so that syntax extensions will work).  Interactive mode
is disabled to suppress initialisation messages from Camlp4Top.
Merge pull request #2 from avsm/fix-under-opam

OPAM requires that the toplevel init checks `OCAML_TOPLEVEL_PATH`