Add configurable post-update journal synchronization via rndc sync -clean.
Separate RFC 2136 update settings from local RNDC control settings.
Run synchronization only after a successful nsupdate request and only when
DYNDNS_RNDC_SYNC_CLEAN is enabled.
Treat RNDC synchronization failures as warnings because the DNS update has
already been accepted and named retains the authoritative state in its journal.