network: dhcp6: introduce UseDelegatedPrefix= setting and enable by default

network: dhcp6: introduce UseDelegatedPrefix= setting and enable by default

Previously, the prefix delegation is enabled when at least one
downstream interfaces request it. But, when the DHCPv6 client on the
upstream interface is configured, some downstream interfaces may not
exist yet, nor have .network file assigned.

Also, if a system has thousands of interfaces, then the previous logic
introduce O(n^2) search.

This makes the prefix delegation is always enabled, except when it is
explicitly disabled. Hopefully, that should not break anything, as the
DHCPv6 server should ignore the prefix delegation request if the server
do not have any prefix to delegate.