Add a new configuration option to set how the checkds method should work. Acceptable values are 'yes', 'no', and 'explicit'.
When set to 'yes', the checkds method is to lookup the parental agents by querying the NS records of the parent zone.
When set to 'no', no checkds method is enabled. Users should run the 'rndc checkds' command to signal that DS records are published and withdrawn.
When set to 'explicit', the parental agents are explicitly configured with the 'parental-agents' configuration option.
Closes #3901 (closed)