chrony: improve hotplug script

chrony: improve hotplug script

- Use the chronyc onoffline command to update state of all sources
  per current routing configuration
- Don't ignore the "ifupdate" action
- Add NTP servers from DHCP for the interface that went up instead of
  the wan4+wan6 interfaces
- Save the servers to files loaded by the sourcedir directive to not
  lose them when chronyd is restarted, and remove them when the
  interface goes down

Signed-off-by: Miroslav Lichvar <mlichvar0@gmail.com>