unix: Add O_CLOFORK & related constants

[ricardobranco777]

Add O_CLOFORK & related constants.

Also:

• Add FD_RESOLVE_BENEATH for FreeBSD.
• Adapt dup3 accordingly.

These systems implement the new POSIX 2024 O_CLOFORK (close-on-fork) constants:

• FreeBSD 15.0
• NetBSD 11.0
• OpenBSD 7.8
• DragonflyBSD master
• Illumos
• Solaris

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

Message from Florian Lehner:

Patch Set 1:

(1 comment)

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