Restart rdnssd service when ntp updates time to proper value for the first time.

Sometimes after a reboot of my router, I loose IPv6 connectivity after roughly 1 - 2 hours (after my prefix expires).
When I turn of the WAN connection and turn it back on, IPv6 connectivity is restored and stays working seemingly forever.

After taking a look at the log multiple times after multiple reboots, I believe it is caused because of the sudden time change when ntp updates the time from Jan 1st some years ago to the current time. I suspect that the rdnssd and/or dhcp6c services do not like this sudden time change (as they start timers).

This change seems to work for me as I haven't lost connectivity yet (over 12 hours).
The log file indeed shows that rdnssd has been restarted.
>Jan  1 01:00:18 WAN Connection: WAN was restored.
>Jan  1 01:00:25 rc_service: ipv6-up 406:notify_rc start_rdnssd
>...
>Apr 22 06:19:55 rc_service: ntp 476:notify_rc restart_rdnssd

To be able to restart rdnssd, I also have to stop dhcp6c before starting in case it was already running.