ddns-scripts: stash the next check time

ddns-scripts: stash the next check time

Calculating the next check time based on the last update time is not
very accurate if the next check is a large multiple forwards from the
last update time because the cumulative sleeps and wake times are not
exact but best effort of the OS. Other factors including clock-drift
give rise to a larger time discrepancy the further the next update is in
the future.

Stash the next check time which should be quite accurate since it's
only one sleep instance away. This is also for use in the GUI.

Tested on 24.10.2

Signed-off-by: Paul Donald <[email protected]>