DHCP6: Don't remove delegated prefix addresses on start

DHCP6: Don't remove delegated prefix addresses on start

In the event where an interface is first delegated and then
started, we don't want the reading of a lease file to remove the
delegated prefixes or addresses.

As such, we also can't test the actual DHCP6 state when building
routes. Instead, just change the test for a delegated prefix
or not so we can still prefer non delegated routes.

Fixes #333.