net: do not set V4MAPPED on FreeBSD

V4MAPPED is not supported on recent FreeBSD versions, at least on 10.1.
Thus, do not set this flag in net.connect on FreeBSD.

Fixes: https://github.com/joyent/node/issues/8540
Fixes: https://github.com/joyent/node/issues/9204
PR-URL: https://github.com/joyent/node/pull/18204
PR-URL: https://github.com/iojs/io.js/pull/1555


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>