This change allows LookupAddr to use getnameinfo through cgo for working
together with various name services other than DNS.
Fixes#7855.
Change-Id: I5b3b4aefe3d1b904541c3350865734d8cbb1c1c4
Reviewed-on: https://go-review.googlesource.com/3420
Reviewed-by: Ian Lance Taylor <iant@golang.org>
