Update relibc constants for Redox compatibility

[ChihweiLHBird]

Description

Fix incorrect placeholder values and add missing constants for Redox in src/unix/redox/mod.rs, verified against relibc @ 53cc0e5a.

Values corrected (were wrong Linux/placeholder values):

• F_DUPFD_CLOEXEC: crate::F_DUPFD (= 0) → 1030 and FIXME(redox): relibc wrapper removed
• IP_RECVTOS: 2 → 13
• TCP_KEEPIDLE: 1 (was same as TCP_NODELAY) → 4 and FIXME(redox): relibc wrapper removed

Constants added from relibc (were missing):

• IPV6_JOIN_GROUP = 20
• IPV6_LEAVE_GROUP = 21

Sources

All values sourced from relibc 53cc0e5aed5b586dccdaad51032b15c0b3bf960e:

• F_DUPFD_CLOEXEC = 1030: https://gitlab.redox-os.org/redox-os/relibc/-/blob/53cc0e5aed5b586dccdaad51032b15c0b3bf960e/src/header/fcntl/mod.rs#L39
• IP_RECVTOS = 13: https://gitlab.redox-os.org/redox-os/relibc/-/blob/53cc0e5aed5b586dccdaad51032b15c0b3bf960e/src/header/netinet_in/mod.rs#L67
• IPV6_JOIN_GROUP = 20, IPV6_LEAVE_GROUP = 21: https://gitlab.redox-os.org/redox-os/relibc/-/blob/53cc0e5aed5b586dccdaad51032b15c0b3bf960e/src/header/netinet_in/mod.rs#L103
• TCP_KEEPIDLE = 4: https://gitlab.redox-os.org/redox-os/relibc/-/blob/53cc0e5aed5b586dccdaad51032b15c0b3bf960e/src/header/netinet_tcp/mod.rs#L12

Checklist

• 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

[Copilot]

Pull request overview

This PR updates Redox platform constants in src/unix/redox/mod.rs to match the actual values defined in relibc, replacing incorrect Linux placeholder values. Redox is a Unix-like operating system written in Rust that uses relibc instead of glibc. The changes ensure that the libc crate provides correct constant values for Redox, which is critical for proper system call behavior.

Changes:

• Corrected three existing constants that had wrong placeholder values (F_DUPFD_CLOEXEC, IP_RECVTOS, TCP_KEEPIDLE)
• Added two new IPV6 constants (IPV6_JOIN_GROUP, IPV6_LEAVE_GROUP) as aliases for existing membership constants
• Removed FIXME comments for corrected constants that now have verified relibc values

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/unix/redox/mod.rs	Corrected F_DUPFD_CLOEXEC from 0 to 1030, IP_RECVTOS from 2 to 13, TCP_KEEPIDLE from 1 to 4, and added IPV6_JOIN_GROUP (20) and IPV6_LEAVE_GROUP (21) constants with FIXME comments removed
libc-test/semver/redox.txt	Added three new public constants to semver tracking: F_DUPFD_CLOEXEC, IPV6_JOIN_GROUP, and IPV6_LEAVE_GROUP

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