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.
(cherry picked from commit 313de3a7e2)
The keyword 'unlimited' can be used instead of PT0S which means the
same but is more comprehensible for users.
Also fix some redundant "none" parameters in the kasp test.