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

Inizia dai punti di ingresso fcntl.fcntl e fcntl.ioctl, confrontando il modo in cui ioctl gestisce mutate_flag con il comportamento attuale di fcntl. Controlla i test fcntl esistenti e le aspettative del buffer protocol; il lavoro è completo quando fcntl.fcntl supporta il parametro proposto e i test coprono la ricezione del buffer modificato senza richiedere struct parsing.