Add `mutate_flag` parameter for `fcntl.fcntl`

Feature or enhancement

Proposal:

fcntl.ioctl has a mutate_flag parameter. When set to True, it will pass the arg buffer to the underlying ioctl syscall directly. This is useful for receiving the result which will be written to the arg buffer.

However, fcntl.fcntl doesn't have this parameter, and it only returns bytes as the modified arg buffer. Users need to parse the bytes using the struct module, even if they have a well-defined structure that already implements the write buffer protocol.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

fcntl.fcntl と fcntl.ioctl のエントリポイントから始め、ioctl が mutate_flag をどのように処理するかを、現在の fcntl の動作と比較してください。既存の fcntl テストと buffer protocol に関する期待を確認してください。fcntl.fcntl が提案されたパラメータをサポートし、struct parsing を必要とせずに変更された buffer を受け取るケースをテストでカバーできれば、作業は完了です。