Avoid calls to (*os.File).Fd() when calling ioctl

[WeidiDeng]

Like #214 , but use unsafe.Pointer as a parameter to avoid the use of reflect.

[WeidiDeng]

@creack Do you have time to review this pr? I'd like to use the non blocking operation without manually calling syscall.SetNonblock.

[creack]

Will try to take a look over the weekend but may be the weekend after that.

[WeidiDeng]

@creack Do you have time to review this pr?

[WeidiDeng]

I think //go:uintptrescapes directive might need very few changes. Will test soon.

[creack]

Thanks @WeidiDeng — really appreciate you sticking with this and following up on the earlier rounds (#214, the //go:uintptrescapes experiment).

The unsafe.Pointer-through-the-stack approach is the right way to satisfy unsafe.Pointer rule (4): the uintptr(ptr) conversion only happens inside the syscall.Syscall argument list, which is what keeps the referenced object alive across the call. That closes a latent GC hazard that's been in the codebase since well before #167.

Merging now. Will hold tagging/release until I've also added a stress test that exercises the SyscallConn().Control() + concurrent close/fork pattern that motivated the original #177 revert — want a more reproducible guard than -count=100. If anyone hits issues against master, please open a follow-up issue.