modules: adding load linked modules feature

- introduced NM_F_LINKED flag to identify linked modules
- setting node_is_initialized after calling V8::Initialize in order to
  make the right decision during initial module registration
- introduced modlist_linked in order to track modules that were
  pre-registered in order to complete it once node is initialized
- completing registration of linked module similarly to the way it's
  done inside DLOpen

PR-URL: https://github.com/iojs/io.js/pull/8
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>