If possible don't use forwarders when priming the resolver.

If we try to fetch a record from cache and need to look into
hints database we assume that the resolver is not primed and
start dns_resolver_prime(). Priming query is supposed to return
NSes for "." in ANSWER section and glue records for them in
ADDITIONAL section, so that we can fill that info in 'regular'
cache and not use hints db anymore.
However, if we're using a forwarder the priming query goes through
it, and if it's configured to return minimal answers we won't get
the addresses of root servers in ADDITIONAL section. Since the
only records for root servers we have are in hints database we'll
try to prime the resolver with every single query.

This patch adds a DNS_FETCHOPT_NOFORWARD flag which avoids using
forwarders if possible (that is if we have forward-first policy).
Using this flag on priming fetch fixes the problem as we get the
proper glue. With forward-only policy the problem is non-existent,
as we'll never ask for root server addresses because we'll never
have a need to query them.

Also added a test to confirm priming queries are not forwarded.
40 jobs for !1296 with 752-disable-forwarders-when-priming in 10 minutes and 47 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #151527
amd64 docker linux
🐞:sid:amd64:precheck

00:00:37

passed #151526
amd64 docker linux
sid:amd64:precheck

00:00:15

 
  Build
passed #151534
amd64 docker linux
asan:sid:amd64:build

00:02:03

passed #151540
amd64 docker linux
bionic:amd64:build

00:01:23

passed #151528
amd64 docker linux
centos6:amd64:build

00:02:26

passed #151529
amd64 docker linux
centos7:amd64:build

00:02:34

passed #151530
amd64 docker linux
clang:stretch:amd64:build

00:02:00

passed #151535
i386 docker linux
clang:stretch:i386:build

00:02:35

passed #151531
amd64 docker linux
jessie:amd64:build

00:02:01

passed #151538
amd64 docker linux
noassert:sid:amd64:build

00:02:25

passed #151537
amd64 docker linux
pkcs11:sid:amd64:build

00:01:07

passed #151533
amd64 docker linux
sid:amd64:build

00:02:04

passed #151536
i386 docker linux
sid:i386:build

00:01:33

passed #151532
amd64 docker linux
stretch:amd64:build

00:01:50

passed #151539
amd64 docker linux
xenial:amd64:build

00:01:48

 
  Test
passed #151541
amd64 docker linux
docs:sid:amd64

00:00:51

passed #151557
amd64 docker linux
install:sid:amd64

00:01:53

passed #151561
amd64 docker linux
noassert:system:sid:amd64

00:06:29

passed #151560
amd64 docker linux
noassert:unit:sid:amd64

00:00:43

passed #151559
amd64 docker linux
pkcs11:system:sid:amd64

00:06:56

passed #151558
amd64 docker linux
pkcs11:unit:sid:amd64

00:00:43

passed #151555
amd64 docker linux
system:asan:sid:amd64

00:07:30

passed #151565
amd64 docker linux
system:bionic:amd64

00:06:39

passed #151550
amd64 docker linux
system:centos6:amd64

00:06:23

passed #151551
amd64 docker linux
system:centos7:amd64

00:06:29

passed #151552
amd64 docker linux
system:jessie:amd64

00:06:39

passed #151554
amd64 docker linux
system:sid:amd64

00:06:58

passed #151556
i386 docker linux
system:sid:i386

00:06:21

passed #151553
amd64 docker linux
system:stretch:amd64

00:06:27

passed #151563
amd64 docker linux
system:xenial:amd64

00:06:47

passed #151547
amd64 docker linux
unit:asan:sid:amd64

00:00:53

passed #151564
amd64 docker linux
unit:bionic:amd64

00:01:27

passed #151542
amd64 docker linux
unit:centos6:amd64

00:00:49

passed #151543
amd64 docker linux
unit:centos7:amd64

00:01:25

passed #151548
amd64 docker linux
unit:clang:stretch:amd64

00:00:47

passed #151544
amd64 docker linux
unit:jessie:amd64

00:00:28

passed #151546
amd64 docker linux
unit:sid:amd64

00:00:50

passed #151549
i386 docker linux
unit:sid:i386

00:00:49

passed #151545
amd64 docker linux
unit:stretch:amd64

00:01:24

passed #151562
amd64 docker linux
unit:xenial:amd64

00:00:53