dns: fix V4MAPPED on FreeBSD

[indutny]

see #332

[indutny]

cc @bnoordhuis

[bnoordhuis]

Quick recap: io.js currently sets AI_V4MAPPED flag indiscriminately but it's underspecified what its meaning is for AF_UNSPEC queries (see e.g. this glibc bug report); it "works" on some platforms but fails on FreeBSD and possibly other BSDs.

[tunniclm]

@indutny
Can this be closed in light of #1555? Or is there more to do on this issue?

[indutny]

Yeah. Thanks!