Recognize BSD hosts in Underlying.bsd?

[OskarEichler]

Summary

OS::Underlying.bsd? returns false on FreeBSD, OpenBSD, NetBSD and DragonFly because it only delegates to the macOS predicate. Recognize those host families alongside Darwin.

Reproduction and verification

Checked eight synthetic host_os values. These are platform stubs on macOS, not native BSD execution.

• External focused repro corpus: 8 checks pass against this isolated branch, under rbenv Ruby 4.0.6.
• Existing upstream suite: 36 examples, zero failures before and after the patch (RBENV_VERSION=4.0.6 rbenv exec rspec -Ilib -Ispec).
• Source-only change; no existing tests modified or new test files added, per the contributor's task constraint. The focused corpus lives outside the repository.
• Local verification was on macOS. Other Ruby versions and native operating systems were not executed. Upstream RuboCop configuration cannot load its legacy rubocop-lts1_8.yml, including after installing current rubocop-lts; lint status is not represented as passing.

Breaking-change notes

No API removal; affected BSD hosts now correctly return true. Darwin remains true and Linux, Solaris and Windows remain false.