Add option to automatically update schedule frequency

The default auto-update schedule for FortiGuard packages has been updated. Previously, the frequency was a reoccurring random interval within two hours. Starting in 7.0, the frequency is automatic, and the update interval is calculated based on the model and percentage of valid subscriptions. The update interval is within one hour.

config system autoupdate schedule
    set frequency {every | daily | weekly | automatic}
end

For example, an FG-501E has 78% valid contracts. Based on this device model, FortiOS calculates the update schedule to be every 10 minutes. If you verify the system event logs (ID 0100041000), they are generated approximately every 10 minutes.