Implement support for NSEC3 in dnssec-policy.  Store the configuration
in kasp objects. When configuring a zone, call 'dns_zone_setnsec3param'
to queue an nsec3param event. This will ensure that any previous
chains will be removed and a chain according to the dnssec-policy is
created.

Add tests for dnssec-policy zones that uses the new 'nsec3param'
option, as well as changing to new values, changing to NSEC, and
changing from NSEC.

(cherry picked from commit 114af58e)