handle_wrap: IsRefed -> Unrefed, no isAlive check

This fixes my perceived usability issues with 7d8882ba. Which, at the
time of writing, has not landed in any release except v6 RCs. This
should not be considered a breaking change due to that.

It is useful if you have a handle, even if it has been closed, to be
able to inspect whether that handle was unrefed or not. As such, this
renames the method accordingly. If people need to check a handle's
aliveness, that is a separate API we should consider exposing.

Refs: https://github.com/nodejs/node/pull/5834
PR-URL: https://github.com/nodejs/node/pull/6204


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>