examples: handle more actions in NM dispatcher script

examples: handle more actions in NM dispatcher script

Run the chronyc onoffline command also when the connectivity-change
and dhcp6-change actions are reported by the NetworkManager dispatcher.

The latter should not be necessary, but there currently doesn't seem to
be any action for IPv6 becoming routable after duplicate address
detection, so at least in networks using DHCPv6, IPv6 NTP servers should
not be stuck in the offline state from a previously reported action.