networkd-dhcp6: Set one unreachable route per DHCPv6 delegated prefix

networkd-dhcp6: Set one unreachable route per DHCPv6 delegated prefix

Instead of setting many small unreachable routes for each of the /64
subnets that were not distributed between the links requesting delegated
prefixes, set one unreachable route for the size of the delegated
prefix. Each subnet asssigned to a downstream link will add a routable
subnet for that link, and as the subnet assigned to the downstream link
has a longer prefix than the whole delegated prefix, the downstream
link subnet routes are preferred over the unroutable delegated one.
The unreachable route is not added when the delegated prefix is exactly
a /64 as the prefix size cannot be used to sort out the order of routing
into a bigger blocking subnet with the smaller /64 punching routable
"holes" into it.

When stopping the DHCPv6 client, the unroutable delegated prefix is
removed before the downstream link prefixes are unassigned.