In general, these functions cannot behave correctly when given a
hostname, because a hostname may represent multiple IP addresses, and
first(isIPv4) chooses at most one.

Updates #9334

Change-Id: Icfb629f84af4d976476385a3071270253c0000b1
Reviewed-on: https://go-review.googlesource.com/31931Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>