inet6: emit RTM_NEWADDR messages on address flag changes.

inet6: emit RTM_NEWADDR messages on address flag changes.

We no longer emit RTM_NEWADDR right away when adding a new address
and starting DAD. Instead the result of the DAD will be emitted.

Some minor fixes from NetBSD are also brought in, such as
starting DAD when addresses are no longer detached and the
tentative state not being added when address lifetime is
extended.

Taken-from: NetBSD
Reviewed-by: sephe