In Go 1.8, almost all the platforms except NaCl provide network
interface and address identification and applications can use IPv4
limited or directed broadcast addresses appropriately.

Fixes #18176.

Change-Id: Ie5de834d19c0aaeb4128a3ca655f6c4c9ae5e501
Reviewed-on: https://go-review.googlesource.com/34435Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>