ICMP6 RA: recognise PIO PD Preferred flag

[bonktree]

This is a 1-bit flag in the Prefix Information RA option defined by RFC9762. If a compliant RA-accepting node receives a PIO with the P flag set, it is expected to ignore the A flag, if set, and make use of DHCPv6 Prefix Delegation to obtain a prefix and assign addresses from that prefix as described in RFC9663, not individual addresses from the PIO-specified prefix.