Do not store unused ICMPv6 Option PREFIX_INFORMATION

Do not store unused ICMPv6 Option PREFIX_INFORMATION

Dead since 2017 sys/netinet6/nd6_rtr.c r1.163
    Remove sending of router solicitations and processing of router
    advertisements from the kernel. It's handled by slaacd(8) these days.

sysctl(2) net.inet6.icmp6.nd6_debug does not warn about it like it does
for, e.g., duplicate MTU options, so don't do anything with this option.

Remove access macros for other unused options while here.
Eventually, union nd_opts should be removed completely.
All under _KERNEL.

tcpdump(8)/rad(8)/slaacd(8) keep showing/sending/receiving this option when
running this diff on both router and client.

OK claudio