rc: Fix const correctness.
dhcpv6: Request a /56 IA_PD prefix
dhcpv6: Make sla-len config optional
dhcpv6: Make sla-id config optional
dhcpv6: Don't set sla-len
dhcpv6: pass IA_PD prefix to dhcpv6 state script
rc: Log IA_PD prefix passed to dhcp6 state script