modules: adding load linked modules feature

 *reapplying [PR from node-fwd](https://github.com/node-forward/node/pull/49)*

- 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