GH-73991: Use same signature for `shutil._rmtree_[un]safe()`. (#120517)

Preparatory work for moving `_rmtree_unsafe()` and `_rmtree_safe_fd()` to
`pathlib._os` so that they can be used from both `shutil` and `pathlib`.

Move implementation-specific setup from `rmtree()` into the safe/unsafe
functions, and give them the same signature `(path, dir_fd, onexc)`.

In the tests, mock `os.open` rather than `_rmtree_safe_fd()` to ensure the
FD-based walk is used, and replace a couple references to
`shutil._use_fd_functions` with `shutil.rmtree.avoids_symlink_attacks`
(which has the same value).

No change of behaviour.