Followup to (#2281)

* Various file moves in the middle end: this is the first stage of improving separation between the middle end and backend.
* Creation of file_formats/ directory (with associated file moves) to hold the definitions of compilation artifact formats.
* Creation of lambda/ directory (with associated file moves) to hold Lambda language definition files, transformation passes and construction passes from Typedtree.
* Disable (hopefully temporarily) dynlink, debugger and ocamldoc for the dune build.

When running a script with "ocaml foo.ml", the toplevel needs to
run foo.ml with a different Sys.argv than the initial value, since
foo.ml must not see the initial "ocaml" argument.

Previously, this was done with Obj.truncate to shorten the Sys.argv
array. This patch changes it by introducing a primitive %sys_argv.
Uses of this primitive expand to a call to a new C primitive, which
returns the argv array (and can be modified by the toplevel).

After consultation on the core developers' list I am proposing this patch to remove support for compiler plugins.

The main motivations for removing compiler plugins are:
- They are a potential security risk.
 - They increase the complexity of the build system and make maintenance of the Dynlink libraries more difficult (although actually, this complexity could probably be reduced after #2268 is merged).
 - Many applications of plugins should be able to be expressed by building custom compiler drivers that link against compilerlibs.

* Remove compiler plugins and hooks
* Add new function Dynlink.unsafe_get_global_symbol but keep it outside the documented API.
* Remove otherlibs/dynlink/nodynlink.ml
* Update Changes

* Delete the deprecated vmthreads library

It was deprecated in 4.08.

* Remove the byte/native argument of init_path

It is no longer necessary.

* Error out when passing --{enable,disable}-vmthreads to ./configure

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

- Add a Load_path module which caches files lookup

- Instead of falling back to the external environment, allow to
  declare in the environment that a module comes from the external
  world. This allows persistent structures to shadows non-persistent
  ones