toplevel: allows the extension of 'genprintval.ml' with parameterised printer.

From: Pierre Chambart <pierre.chambart@ocamlpro.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15632 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
toplevel: extends #install_printer to accept parameterised printers

From: Pierre Chambart <pierre.chambart@ocamlpro.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15633 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
toplevel: generic printers are only allowed with 'new' printer style.

From: Pierre Chambart <pierre.chambart@ocamlpro.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15634 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
toplevel: be explicit about what path are used for

From: Pierre Chambart <pierre.chambart@ocamlpro.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15635 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02