Update the ARM to mention the new built-in "insecure" policy. Update
the DNSSEC guide recipe "Revert to unsigned" to add the additional
step of reconfiguring the zone to "insecure" (instead of immediately
set it to "none").
Add a new option 'purge-keys' to 'dnssec-policy' that will purge key
files for deleted keys. The option determines how long key files
should be retained prior to removing the corresponding files from
disk.
If set to 0, the option is disabled and 'named' will not remove key
files from disk.
