net: export isIPv4, isIPv6 directly from cares

The function objects encapsulating `isIPv4` and `isIPv6` are not
necessary. They can be directly exposed from `cares`.

PR-URL: https://github.com/nodejs/node/pull/7481


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>