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

Start at the fcntl.fcntl and fcntl.ioctl entry points, comparing how ioctl handles mutate_flag with the current fcntl behavior. Check the existing fcntl tests and the buffer-protocol expectations; the work is done when fcntl.fcntl supports the proposed parameter and tests cover receiving the modified buffer without requiring struct parsing.