Add a new built-in policy "insecure", to be used to gracefully unsign
a zone. Previously you could just remove the 'dnssec-policy'
configuration from your zone statement, or remove it.

The built-in policy "none" (or not configured) now actually means
no DNSSEC maintenance for the corresponding zone. So if you
immediately reconfigure your zone from whatever policy to "none",
your zone will temporarily be seen as bogus by validating resolvers.

This means we can remove the functions 'dns_zone_use_kasp()' and
'dns_zone_secure_to_insecure()' again. We also no longer have to
check for the existence of key state files to figure out if a zone
is transitioning to insecure.