nd6: Add router address and DHCPv6-PD flags in PIO headers

[pouria]

Summary

Make R-bit per RFC 6275 8.3 and P-bit per RFC 9762 7.1 in
Prefix Information option available to userland for future implementations.

RFC 9762 7.1: For each interface, the client MUST keep a list of every prefix
that was received from a PIO with the P flag set and currently has a non-zero
preferred lifetime.

RFC 6275 8.3: All IPv6 routers, *even those not serving as a home agent for Mobile IPv6*:
Each router SHOULD include at least one prefix with the Router
Address (R) bit set and with its full IP address in its Router Advertisements.

[pouria]

@ae, that's where I said we would probably have conflict over the reserved flags.

[pouria]

I didn't include the sys/netinet6/nd6_rtr.c part yet, so there is no dependency with other ndp revisions.
But I want to push them at once. (ofc, if approved)

[glebius]

Not an expert. Looks good code wise.