use sys/unix instead of syscall

Suggestion from:
https://github.com/v2fly/v2ray-core/pull/1395#issuecomment-974761647

transport/internet/sockopt_darwin.go

@@ -1,7 +1,7 @@
 package internet
 
 import (
-	"syscall"
+	"golang.org/x/sys/unix"
 )
 
 const (
@@ -18,20 +18,20 @@ func applyOutboundSocketOptions(network string, address string, fd uintptr, conf
 	if isTCPSocket(network) {
 		switch config.Tfo {
 		case SocketConfig_Enable:
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_FASTOPEN, TCP_FASTOPEN_CLIENT); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN, unix.TCP_FASTOPEN_CLIENT); err != nil {
 				return err
 			}
 		case SocketConfig_Disable:
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_FASTOPEN, 0); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN, 0); err != nil {
 				return err
 			}
 		}
 
 		if config.TcpKeepAliveInterval > 0 {
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
 				return newError("failed to set TCP_KEEPINTVL", err)
 			}
-			if err := syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_KEEPALIVE, 1); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_KEEPALIVE, 1); err != nil {
 				return newError("failed to set SO_KEEPALIVE", err)
 			}
 		}
@@ -44,19 +44,19 @@ func applyInboundSocketOptions(network string, fd uintptr, config *SocketConfig)
 	if isTCPSocket(network) {
 		switch config.Tfo {
 		case SocketConfig_Enable:
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_FASTOPEN, TCP_FASTOPEN_SERVER); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN, TCP_FASTOPEN_SERVER); err != nil {
 				return err
 			}
 		case SocketConfig_Disable:
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_FASTOPEN, 0); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_FASTOPEN, 0); err != nil {
 				return err
 			}
 		}
 		if config.TcpKeepAliveInterval > 0 {
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.IPPROTO_TCP, unix.TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
 				return newError("failed to set TCP_KEEPINTVL", err)
 			}
-			if err := syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_KEEPALIVE, 1); err != nil {
+			if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_KEEPALIVE, 1); err != nil {
 				return newError("failed to set SO_KEEPALIVE", err)
 			}
 		}