net: refactor overloaded argument handling

* Make normalizeArgs return either [options, null] or [options, cb]
  (the second element won't be undefined anymore) and avoid OOB read
* Use Socket.prototype.connect.call instead of .apply when the number
  of arguments is certain(returned by normalizeArgs).
* Rename some args[i] for readability
* Refactor Server.prototype.listen, separate backlogFromArgs and
  options.backlog, comment the overloading process, refactor control
  flow

PR-URL: https://github.com/nodejs/node/pull/11667


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>