Lwt_unix should use `caml_unix_check_path` or `caml_string_is_c_safe` for C stubs that take a string as input

For the Unix module this got fixed in OCaml 4.03 https://github.com/ocaml/ocaml/pull/227, but Lwt still allows them.

```
Lwt_unix.openfile "/tmp/test\x00test" [Unix.O_CREAT;Unix.O_RDWR] 0;;
- : Lwt_unix.file_descr = <abstr>
```

```
Unix.openfile "/tmp/foox\x00bar" [Unix.O_CREAT;Unix.O_RDWR] 0;;
Exception: Unix.Unix_error(Unix.ENOENT, "open", "/tmp/foox\000bar")
```
