lib,test: deprecate _linklist

Deprecate _linklist and add test to confirm internal linklist and
public _linklist are the same.

PR-URL: https://github.com/nodejs/node/pull/3078


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>