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.

[Skyb0rg007]

For now, I simply guard the offending struct definitions with #[cfg(...)]. I still think the src/unix/mod.rs file is the right place (instead of one of the platform-specific subdirectories) because these structures and definitions are specified by network protocols. I think the three "complete" solutions would be:

1. Add struct ip6_addr_packed without an alignment attribute. Use that for these fields.
2. Use [u8; 16] instead of ip6_addr for these platforms.
3. Break alignment compatibility with these structures on the offending platforms. From the FreeBSD documentation, the use of __packed is used in network protocol code to avoid padding bytes, not to allow non-aligned data.

Otherwise I think this PR is ready to review.

[Skyb0rg007]

I think GNU Hurd (due to using glibc) and Fuchsia (due to Zircon) could add ICMP6_FILTER = 1. It's also available on Solaris (docs). I don't have a way to test these platforms though.