Improve handling of future leap-seconds via NTP
Paul Boven reports:
"On a switch, does the unix (busybox) time correctly handle the leap
second? The (busybox) ntp is only ever run at boot time, and clearly
won't receive any leap second announcements after that, if they are even
implemented in that ntpd."
In the grand master (and free running master) ntp shall run periodically
to get the information about leap seconds.
We should check whether the ntp notifies the linux kernel about leap
seconds and whether it tracks the offset between tai and utc (field tai
of struct timex).