unix: add GetsockoptUint64 and SetsockoptUint64

The AF_VSOCK address family on Linux uses unsigned long long (uint64
in Go) values for getting and setting some socket options.

Change-Id: If28f6a5d04c0c988b7cbcaf2bf8d184a69329882
Reviewed-on: https://go-review.googlesource.com/c/sys/+/169959
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix.go

@@ -294,6 +294,13 @@ func GetsockoptTimeval(fd, level, opt int) (*Timeval, error) {
 	return &tv, err
 }
 
+func GetsockoptUint64(fd, level, opt int) (value uint64, err error) {
+	var n uint64
+	vallen := _Socklen(8)
+	err = getsockopt(fd, level, opt, unsafe.Pointer(&n), &vallen)
+	return n, err
+}
+
 func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	var rsa RawSockaddrAny
 	var len _Socklen = SizeofSockaddrAny
@@ -351,6 +358,10 @@ func SetsockoptTimeval(fd, level, opt int, tv *Timeval) (err error) {
 	return setsockopt(fd, level, opt, unsafe.Pointer(tv), unsafe.Sizeof(*tv))
 }
 
+func SetsockoptUint64(fd, level, opt int, value uint64) (err error) {
+	return setsockopt(fd, level, opt, unsafe.Pointer(&value), 8)
+}
+
 func Socket(domain, typ, proto int) (fd int, err error) {
 	if domain == AF_INET6 && SocketDisableIPv6 {
 		return -1, EAFNOSUPPORT