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

Comienza por los puntos de entrada fcntl.fcntl y fcntl.ioctl, comparando cómo ioctl gestiona mutate_flag con el comportamiento actual de fcntl. Revisa las pruebas existentes de fcntl y las expectativas del buffer protocol; el trabajo estará terminado cuando fcntl.fcntl admita el parámetro propuesto y las pruebas cubran la recepción del buffer modificado sin requerir struct parsing.