Restart rdnssd when ntp sets time after reboot

Reapplying #604/#613: when the time updates to the proper value and dhcp6c is already started by rdnssd, dhcp6c may fail renewing its lease, causing IPv6 to stop working.

In wan.c, start_rdnssd is outside the #ifndef RTCONFIG_DNSMASQ6, so I have added the code outside the ifdef as well.

In 376.1071, start_radvd would call stop_radvd, so I have also included those changes in services.c

In start_dhcp6c, an existing instance will be stopped to prevent multiple instances listening and causing conflicts.