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

從 fcntl.fcntl 和 fcntl.ioctl 入口點開始，比較 ioctl 如何處理 mutate_flag 與目前 fcntl 行為之間的差異。檢查現有的 fcntl 測試和 buffer protocol 的預期；當 fcntl.fcntl 支援所提議的參數，且測試涵蓋接收修改後的 buffer 而不需要 struct parsing 時，這項工作就完成了。