Introduce dnssec-policy configuration
This commit introduces the initial `dnssec-policy` configuration statement. It has an initial set of options to deal with signature and key maintenance. Add some checks to ensure that dnssec-policy is configured at the right locations, and that policies referenced to in zone statements actually exist. Add some checks that when a user adds the new `dnssec-policy` configuration, it will no longer contain existing DNSSEC configuration options. Specifically: `inline-signing`, `auto-dnssec`, `dnssec-dnskey-kskonly`, `dnssec-secure-to-insecure`, `update-check-ksk`, `dnssec-update-mode`, `dnskey-sig-validity`, and `sig-validity-interval`. Test a good kasp configuration, and some bad configurations.