gh-59705: Set OS thread name when Thread.name is changed

[vstinner]

• Issue: Python should support exporting thread names to the OS #59705

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📚 Documentation preview 📚: https://cpython-previews--127702.org.readthedocs.build/

[vstinner]

@serhiy-storchaka @encukou: I propose to update the OS thread name when Thread.name of a running thread is changed, but only if it's the current thread.

The problem is that on macOS, pthread_setname_np() has a single argument: the thread name.

[serhiy-storchaka]

LGTM. There are questions for implementation (What about empty name? Is not it dangerous to silently ignore setting the OS name when set name from other thread?), but we can only answer them after users started using this feature.

[vstinner]

Merged, thanks for reviews.