Fix Issue 23456 - OpenBSD: Add waitid support

[ibara]

As stated, we support waitid(2) now.
https://marc.info/?l=openbsd-cvs&m=166671373628366&w=2

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23456	enhancement	OpenBSD: Add waitid support

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#14612"

[ibuclaw]

It is might be nice if we could gate this by version, but not necessary.

[dkorpel]

What does you mean with "gate this by version"?

[ibara]

At least for the OpenBSD packages, there will never be a case where someone installs a package of DRuntime with waitid support but not have an OpenBSD system that does not have waitid support. If the user doesn't use the package, they are on their own, and should know better.