net: make `isIPv4` and `isIPv6` more efficient

`isIPv4` and `isIPv6` are implemented on top of `isIP`, which in turn
checks the sting for being both IPv4 and IPv6, which can be inefficient
in some scenarios. This commit makes them use `uv_inet_pton` directly
instead.

PR-URL: https://github.com/nodejs/node/pull/5478


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>