gh-137109: refactor warning about threads when forking (#141438)

gh-137109: refactor warning about threads when forking (#141438)

* gh-137109: refactor warning about threads when forking

This splits the OS API specific functionality to get the number of threads out
from the fallback Python method and warning raising code itself.  This way the
OS APIs can be queried before we've run
`os.register_at_fork(after_in_parent=...)` registered functions which
themselves may (re)start threads that would otherwise be detected.

This is best effort.  If the OS APIs are either unavailable or fail, the
warning generating code still falls back to looking at the Python threading
state after the CPython interpreter world has been restarted and the
after_in_parent calls have been made.  The common case for most Linux and macOS
environments should work today.

This also lines up with the existing TODO refactoring, we may choose to expose
this API to get the number of OS threads in the `os` module in the future.

* NEWS entry

* avoid "function-prototype" compiler warning?