tools: update ESLint, fix unused vars bug

Update ESLint to 3.0.0. This includes an enhancement to `no-unused-vars`
such that it finds a few instances in our code base that it did not find
previously (fixed in previous commits readying this for landing).

PR-URL: https://github.com/nodejs/node/pull/7601


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>