Allow running without updating system clock

[elipsion]

For $REASON I'm stuck in an environment without CAP_SYS_TIME, but still want to provide an NTP service.

I'd like a way to tell ntp-daemon to not update the system time after reaching a synchronized state, but rather just use that delta as a known offset from the system clock when reporting time to clients.

I think the most logical place to put this would be in [synchronization] as update-system-clock = bool (true)