Introduce dnssec-policy configuration

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.