net: refactor redeclared variables

`lib/net.js` contained four variables that were redeclared with `var` in
the same scope. This change refactors those redeclarations so they are
scoped properly.

PR-URL: https://github.com/nodejs/node/pull/4963


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>