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

Beginne bei den Einstiegspunkten fcntl.fcntl und fcntl.ioctl und vergleiche, wie ioctl mutate_flag behandelt, mit dem aktuellen Verhalten von fcntl. Prüfe die vorhandenen fcntl-Tests und die Erwartungen an das buffer protocol; die Arbeit ist abgeschlossen, wenn fcntl.fcntl den vorgeschlagenen Parameter unterstützt und Tests das Empfangen des modifizierten Buffers abdecken, ohne struct parsing zu erfordern.