net: there are no invalid domain names anymore

net: there are no invalid domain names anymore

The Go resolver reports invalid domain name for '!!!.local',
but that is allowed by multicast DNS. In general we can't predict
what future relaxations might come along, and libc resolvers
do not distinguish 'no such host' from 'invalid name', so stop
making that distinction here too. Always use 'no such host'.

Fixes #12421.

Change-Id: I8f22604767ec9e270434e483da52b337833bad71
Reviewed-on: https://go-review.googlesource.com/31468
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>