syscall.SetsockoptInt cannot work on Windows

[YoshieraHuang]

When I build this project on Windows, this error appears:

cannot use int(file.Fd()) (type int) as type syscall.Handle in argument to syscall.GetsockoptInt

Google says that GetsockoptInt is a very Unix thing and not compatible with Windows. Codes for GetsockoptInt in golang for Windows is like this:

func GetsockoptInt(fd Handle, level, opt int) (int, error) { return -1, EWINDOWS }

[YoshieraHuang]

syscall package for Windows define a new type syscall.Handle and it is an equivalence to uintptr. The first argument of GetsockoptInt and SetsocketoptInt functions is syscall.Handle in Windows, not uintptr in Linux.

[quentinmit]

Unfortunately I don't really have any good way to test this package on Windows; in that particular case, however, it looks like GetsockoptInt is now implemented in the golang.org/x/sys/windows package that replaced syscall. While Windows and Unix are often very different, Windows adopted the Unix socket API, so getsockopt does indeed exist on windows.

I think we'd need to take all the calls to GetsockoptInt and SetsockoptInt and put them into build tag-controlled files that import unix or windows as appropriate. I'm probably unlikely to do that myself but I'm happy to take a PR.