net: move isLegalPort to internal/net

isLegalPort can be used in more places than just net.js. This change
moves it to a new internal net module in preparation for using it in
the dns module.

PR-URL: https://github.com/nodejs/node/pull/4882


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>