3421. [bug] Named loops when re-signing if all keys are offline.

3421.   [bug]           Named loops when re-signing if all keys are offline.
                        [RT #31916]

Squashed commit of the following:

commit f47af0ca6793687b9c8d08fd44b0c091ba5a4f9a
Author: Mark Andrews <marka@isc.org>
Date:   Wed Nov 21 17:45:21 2012 +1100

    dns_dns_zonediff_t -> dns_zonediff_t, clarify comment

commit 344edefc3ee90856a7ff990abe7971925ba843b2
Author: Mark Andrews <marka@isc.org>
Date:   Tue Nov 20 13:12:26 2012 +1100

    commit the zone changes if a keep was marked as being offline

commit cad2c2446ebfc20b6d8c4f6dd0d6596d7106cc0f
Author: Mark Andrews <marka@isc.org>
Date:   Tue Nov 20 13:08:29 2012 +1100

    check for looping when re-signing expiring.example