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

从 fcntl.fcntl 和 fcntl.ioctl 入口点开始，比较 ioctl 如何处理 mutate_flag 与当前 fcntl 行为之间的差异。检查现有的 fcntl 测试和 buffer protocol 的预期；当 fcntl.fcntl 支持所提议的参数，并且测试覆盖接收修改后的 buffer 而不需要 struct parsing 时，这项工作就完成了。