For inline-signed zones, the value of "ixfr-from-differences" is
hardcoded to:

  - "yes" for the raw version of the zone,
  - "no" for the signed version of the zone.

In other words, any user-provided "ixfr-from-differences" setting is
effectively ignored for an inline-signed zone.  Ensure the user is aware
of that by adding a note to the ARM and logging a message when an
"ixfr-from-differences" option is found at the zone level.

(cherry picked from commit 087157d1)