net: allow port 0 in connect()

The added validation allows non-negative numbers and numeric
strings. All other values result in a thrown exception.

Fixes: https://github.com/joyent/node/issues/9194
PR-URL: https://github.com/joyent/node/pull/9268


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@users.noreply.github.com>