lib/resolve answer_finalize: don't SERVFAIL bogus +cd

As kresd works now, typically we do not know whether these records are
bogus, as with +cd we do not attempt validation.  Still, it's possible
that we have those records in cache from an occasion without +cd, in
which case we know they're bogus and this regression happened.

The potential impact of this issue seems minimal.
27 jobs for <span class="ref-name">cd-servfail</span> in 39 minutes and 51 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #293548
docker linux amd64
archive

00:00:37

passed #293549
docker linux amd64
build

00:00:45

passed #293550
docker linux amd64
build-asan

00:00:45

passed #293551
docker
kres-gen

00:00:45

 
  Test
passed #293552
docker delayed
build:darwin

00:01:24

passed #293553
docker linux amd64
deckard

00:02:17

passed #293554
docker delayed
doc

00:00:56

passed #293555
dind
docker

00:02:38

passed #293556
docker delayed
lint:other

00:00:54

passed #293557
docker linux amd64 delayed
lint:pedantic

00:01:06

passed #293558
docker linux amd64
lint:scan-build

00:02:16

passed #293559
docker linux amd64
lint:tidy

00:02:53

passed #293560
privileged
pkg:epel-7

00:02:37

passed #293561
privileged
pkg:fedora-29

00:03:51

passed #293562
docker linux amd64
respdiff:basic

00:02:23

passed #293563
docker linux amd64 delayed
test:valgrind

00:01:50

 
  Extended
passed #293565
respdiff
fwd-tls6-kresd.udp6

00:24:30

passed #293566
respdiff
fwd-udp6-kresd.udp6

00:34:31

failed #293570
respdiff
fwd-udp6-unbound.tcp6

00:27:51

passed #293571
respdiff
fwd-udp6-unbound.tls6

00:28:21

passed #293569
respdiff
fwd-udp6-unbound.udp6

00:18:21

passed #293568
respdiff
iter.tls6

00:29:00

passed #293567
respdiff
iter.udp6

00:15:21

passed #293564
docker linux amd64
pytests

00:04:09

passed #293572
respdiff
resperf:fwd-tls6.udp-asan

00:18:16

passed #293573
respdiff
resperf:fwd-udp6.udp-asan

00:14:46

passed #293574
respdiff
resperf:iter.udp-asan

00:27:25

 
Name Stage Failure
failed
fwd-udp6-unbound.tcp6 Extended
$ cp -t . respdiff_commitdir/*$RESPDIFF_TEST*.png ||:
$ cat respdiff_commitdir/$RESPDIFF_TEST/*histogram.tar.gz | tar -xf - -i ||:
Uploading artifacts...
Runtime platform arch=amd64 os=linux pid=86139 revision=de7731dd version=12.1.0
./j*: found 18 matching files
./*.png: found 2 matching files
./*histogram/*: found 6 matching files
Uploading artifacts to coordinator... ok id=293570 responseStatus=201 Created token=x6fnc4vB
ERROR: Job failed: exit status 1