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 #8540 and #9204.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/joyent/node/pull/18204