[release-branch.go1.7] net: don't avoid resolving .local addresses

[release-branch.go1.7] net: don't avoid resolving .local addresses

.local addresses are used by things like Kubernetes and Weave DNS; Go
should not avoid resolving them.

This is a partial revert of https://golang.org/cl/21328 which was too
strict of an interpretation of RFC 6762.

Fixes #16739

Change-Id: I349415b4eab5d61240dd18217bd95dc7d2105cd5
Reviewed-on: https://go-review.googlesource.com/27250
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/28632