Fix Issue 22454 - OpenBSD: Add prototypes for pthread_np.h

[ibara]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22454	enhancement	OpenBSD: Add prototypes for pthread_np.h

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 + druntime#3605"

[ibara]

Spurious dlang-community/dfmt test error?

[kinke]

The other BSD variants don't publicly import this module; I guess non-publicly importing stack_t alone would be better wrt. consistent namespace pollution.

[ibara]

I do plan to use this module in one place in druntime after this is committed.
I switched public import core.sys.posix.signal; // for stack_t to private import core.sys.posix.signal : stack_t; in a force push.

[kinke]

Thx - private is the default and so not needed; there are only half a dozen other occurrences of private import in druntime/Phobos, probably ancient.

[ibara]

Alright; I'll remove the private keyword. Thanks.

[ibara]

(Done)

[kinke]

[Wrt. dfmt, that's totally unrelated, see https://github.com/dlang-community/dfmt/pull/544.]