net: strict checking for internal/net isLegalPort

Add stricter testing for the isLegalPort method in internal/net.
This ensures that odd inputs such as isLegalPort(true) and
isLegalPort([1]) aren't acceptable as valid port inputs.

PR-URL: https://github.com/nodejs/node/pull/5733


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>