bsd: add IN6_IFF_* and SIOCGIFAFLAG_IN6 on apple and freebsd

[sbogomolov]

Description

Add the IN6_IFF_* address flags and the SIOCGIFAFLAG_IN6 ioctl from
netinet6/in6_var.h on Apple and FreeBSD. The flag sets differ a bit: Apple
defines DYNAMIC, OPTIMISTIC, SECURED, CLAT46, and NOPFX that FreeBSD
doesn't, and FreeBSD has PREFER_SOURCE.

Sources

Apple IN6_IFF_* / SIOCGIFAFLAG_IN6 (netinet6/in6_var.h):

• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/netinet6/in6_var.h#L814-L832
• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/netinet6/in6_var.h#L645

FreeBSD IN6_IFF_* / SIOCGIFAFLAG_IN6 (netinet6/in6_var.h):

• https://github.com/freebsd/freebsd-src/blob/17602eb3eae95a53c973a5ceb4fc3134a770e8d6/sys/netinet6/in6_var.h#L470-L480
• https://github.com/freebsd/freebsd-src/blob/17602eb3eae95a53c973a5ceb4fc3134a770e8d6/sys/netinet6/in6_var.h#L431

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 label +stable-nominated

[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.