DHCP6: Keep running regardless of RA state

DHCP6: Keep running regardless of RA state

We should keep DHCP6 running even if routers expire or link changes.
This is noted in RFC3315 18.1.2:
   If the client receives no responses before the message transmission
   process terminates, as described in section 14, the client SHOULD
   continue to use any IP addresses, using the last known
   lifetimes for those addresses, and SHOULD continue to use any other
   previously obtained configuration parameters.