Update named_zone_reusable() so that it does not consider a zone to be
eligible for reuse if its old value of the "mirror" option differs from
the new one. This causes "rndc reconfig" to create a new zone structure
whenever the value of the "mirror" option is changed, which ensures that
the previous zone database is not reused and that flags are properly set
in responses sourced from zones whose "mirror" setting was changed at
runtime.
Replace "type: slave" with "type: mirror" in "rndc zonestatus" output
for mirror zones in order to enable the user to tell a regular slave
zone and a mirror zone apart.
As mirror zone files are verified when they are loaded from disk, verify
journal files as well to ensure invalid data is not used. Reuse the
journals generated during IXFR tests to test this.
Update axfr_commit() so that all incoming versions of a mirror zone
transferred using AXFR are verified before being used. If zone
verification fails, discard the received version of the zone, wait until
the next refresh and retry.
