RFC 2292 struct and constant definitions

[Skyb0rg007]

Description

This PR adds the structs and constants as defined by RFC 2292 - Advanced Sockets API for IPv6.
These constants are available for use in get/setsockopt on supported Unix platforms.
The operations defined on the icmp6_filter struct are defined as C macros, so the implementations are not included here.

Sources

https://github.com/bminor/glibc/blob/04e750e75b73957cf1c791535a3f4319534a52fc/inet/netinet/icmp6.h
https://github.com/kraj/musl/blob/kraj/master/include/netinet/icmp6.h
https://github.com/freebsd/freebsd-src/blob/9ae367d11de8abbdf53884836c9ba30908c5c8db/sys/netinet/icmp6.h

Also in the RFC: https://datatracker.ietf.org/doc/html/rfc2292#section-3.2

Checklist

Unfortunately I could not get the libc-tests working locally, so this PR is marked as draft.

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

[Skyb0rg007]

Alright, I need help. On *BSD platforms (FreeBSD, macOS, etc), struct in6_addr is defined as the union of uint32_t[4], uint16_t[8], and uint8_t[16], so Rust encodes it as [u8; 16] with repr(align(4)).

But the nd_(router|neighbor)_(solicit|advert) structures are marked as __packed (so their C alignment is 1), even though they include a in6_addr. Rust won't allow using repr(packed) on a struct containing a repr(align(...)) attribute, so I'm stuck.

Linux doesn't mark these structs as __packed, so there's no issue there.