Fix a quirky mkeys test failure

Fix a quirky mkeys test failure

The mkeys system test started to fail after introducing support for
zones transitioning to unsigned without going bogus. This is because
there was actually a bug in the code: if you reconfigure a zone and
remove the "auto-dnssec" option, the zone is actually still DNSSEC
maintained. This is because in zoneconf.c there is no call
to 'dns_zone_setkeyopt()' if the configuration option is not used
(cfg_map_get(zoptions, "auto-dnssec", &obj) will return an error).

The mkeys system test implicitly relied on this bug: initially the
root zone is being DNSSEC maintained, then at some point it needs to
reset the root zone in order to prepare for some tests with bad
signatures. Because it needs to inject a bad signature, 'auto-dnssec'
is removed from the configuration.

The test pass but for the wrong reasons:

I:mkeys:reset the root server
I:mkeys:reinitialize trust anchors
I:mkeys:check positive validation (18)

The 'check positive validation' test works because the zone is still
DNSSEC maintained: The DNSSEC records in the signed root zone file on
disk are being ignored.

After fixing the bug/introducing graceful transition to insecure,
the root zone is no longer DNSSEC maintained after the reconfig.

The zone now explicitly needs to be reloaded because otherwise the
'check positive validation' test works against an old version of the
zone (the one with all the revoked keys), and the test will obviously
fail.

(cherry picked from commit 2fc42b598b954f8cbd8a5fc99cc990b15b235b40)