[release-branch.go1.15] net: filter bad names from Lookup functions instead of hard failing

Instead of hard failing on a single bad record, filter the bad records
and return anything valid. This only applies to the methods which can
return multiple records, LookupMX, LookupNS, LookupSRV, and LookupAddr.

When bad results are filtered out, also return an error, indicating
that this filtering has happened.

Updates #46241
Updates #46979
Fixes #47012

Change-Id: I6493e0002beaf89f5a9795333a93605abd30d171
Reviewed-on: https://go-review.googlesource.com/c/go/+/332549


Trust: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
(cherry picked from commit 296ddf2a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/333331
Run-TryBot: Roland Shoemaker <roland@golang.org>