This PR adds support for POSIX O_CLOFORK (close-on-fork) flag. If there's interest I can finish the TODO items.

This is also being done on:

• FreeBSD: Add support for POSIX O_CLOFORK freebsd/freebsd-src#1698
• DragonflyBSD: Add support for POSIX O_CLOFORK  DragonFlyBSD/DragonFlyBSD#28
• NetBSD: Add support for POSIX O_CLOFORK NetBSD/src#53

This may be useful for Golang, Rust, Swift (and other languages):

• os: StartProcess ETXTBSY race on Unix systems golang/go#22315
• Command::spawn on a newly-written file can fail with ETXTBSY due to racing with itself on Unix rust-lang/rust#114554
• Emulate POSIX_SPAWN_CLOEXEC_DEFAULT in fork/exec swiftlang/swift-subprocess#79 (comment)

Illumos' test suite: 663 PASSED, 0 failed or skipped.

TODO

• Integrate the adapted CDDL-licensed oclo tests in the regress directory.
• Adapt current tests that use O_CLOEXEC
• Add FreeBSD dup_test?
• Update manpages

POSIX References:

• O_CLOFORK: https://pubs.opengroup.org/onlinepubs/9799919799/functions/open.html
• FD_CLOFORK: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/fcntl.h.html
• SOCK_CLOFORK: https://pubs.opengroup.org/onlinepubs/9799919799.2024edition/basedefs/sys_socket.h.html
• dup3() with O_CLOFORK : https://pubs.opengroup.org/onlinepubs/9799919799/functions/dup.html

Illumos references:

• Oclo tests: https://github.com/illumos/illumos-gate/tree/master/usr/src/test/os-tests/tests/oclo