Add RFC 3678 multicast group_req/group_source_req and MCAST_*

[sbogomolov]

Description

Add group_req/group_source_req and the MCAST_* socket options for the protocol-independent multicast API (RFC 3678) on Linux, Apple, and FreeBSD. Linux already had the MCAST_* constants. Apple needs #[repr(packed(4))] since the structs are under #pragma pack(4) there. DragonFly doesn't have this API, so it's FreeBSD-only.

Sources

Linux group_req / group_source_req (linux/in.h):

• https://github.com/torvalds/linux/blob/dc59e4fea9d83f03bad6bddf3fa2e52491777482/include/uapi/linux/in.h#L214

Apple group_req / group_source_req / MCAST_* (netinet/in.h):

• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/netinet/in.h#L550
• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/netinet/in.h#L483

FreeBSD group_req / group_source_req / MCAST_* (netinet/in.h):

• https://github.com/freebsd/freebsd-src/blob/9ea13242985dc145b2471cc8f651be87c6a74bfe/sys/netinet/in.h#L580
• https://github.com/freebsd/freebsd-src/blob/9ea13242985dc145b2471cc8f651be87c6a74bfe/sys/netinet/in.h#L515

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are included
• Tested locally (cd libc-test && cargo test --target mytarget)

[rustbot]

Some changes occurred in an Android module

cc @maurer

[tgross35]

@sbogomolov do you actually have a use these constants? Three PRs in the past few hours is a lot.

Also, these PR descriptions look like they may be AI-generated. Please handwrite all PR descriptions and commit messages, and make sure to include the information requested in the PR template.

[sbogomolov]

@sbogomolov do you actually have a use these constants? Three PRs in the past few hours is a lot.

Also, these PR descriptions look like they may be AI-generated. Please handwrite all PR descriptions and commit messages, and make sure to include the information requested in the PR template.

I actually use these, yes :) Not every single constant is used by me, but I thought it would make sense to add related constants together. They are used in my reflector project (mDNS, SSDP/DIAL, WoL). I'm trying to create PRs for all constants / structs that I had to manually define.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.