[release-branch.go1.15] net: verify results from Lookup* are valid domain names

For the methods LookupCNAME, LookupSRV, LookupMX, LookupNS, and
LookupAddr check that the returned domain names are in fact valid DNS
names using the existing isDomainName function.

Thanks to Philipp Jeitner and Haya Shulman from Fraunhofer SIT for
reporting this issue.

Updates #46241
Fixes #46356
Fixes CVE-2021-33195

Change-Id: I47a4f58c031cb752f732e88bbdae7f819f0af4f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/323131


Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
(cherry picked from commit cdcd0284)
Reviewed-on: https://go-review.googlesource.com/c/go/+/323269