tun_netbsd: update to follow tun_openbsd changes

Most of tun_netbsd is from tun_openbsd with some small changes, so we
should follow the changes made in tun_openbsd.

tun/tun_netbsd.go

@@ -6,6 +6,7 @@
 package tun
 
 import (
+    "errors"
 	"fmt"
 	"io/ioutil"
 	"net"
@@ -130,7 +131,7 @@ func CreateTUN(name string, mtu int) (Device, error) {
 	} else {
 		for ifIndex = 0; ifIndex < 256; ifIndex += 1 {
 			tunfile, err = os.OpenFile(fmt.Sprintf("/dev/tun%d", ifIndex), unix.O_RDWR, 0)
-			if err == nil || !errorIsEBUSY(err) {
+			if err == nil || !errors.Is(err, syscall.EBUSY) {
 				break
 			}
 		}
@@ -164,7 +165,7 @@ func CreateTUN(name string, mtu int) (Device, error) {
         uintptr(unsafe.Pointer(&ifheadmode)),
     )
 
-    if errno != 0 || ifheadmode != 1 {
+    if errno != 0 || ifheadmode == 0 {
         tunfile.Close()
         return nil, fmt.Errorf("Unable to validate IFHEAD mode: %v (ifheadmode = %d)", errno, ifheadmode)
     }
@@ -249,19 +250,15 @@ func (tun *NativeTun) Read(buff []byte, offset int) (int, error) {
 		return 0, err
 	default:
 		buff := buff[offset-4:]
-        // buff := buff[offset:]
 		n, err := tun.tunFile.Read(buff[:])
 		if n < 4 {
 			return 0, err
 		}
 		return n - 4, err
-        // return n, err
 	}
 }
 
 func (tun *NativeTun) Write(buff []byte, offset int) (int, error) {
-
-
 	// reserve space for header
 
 	buff = buff[offset-4:]
@@ -289,15 +286,17 @@ func (tun *NativeTun) Flush() error {
 }
 
 func (tun *NativeTun) Close() error {
-	var err2 error
-	err1 := tun.tunFile.Close()
-	if tun.routeSocket != -1 {
-		unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
-		err2 = unix.Close(tun.routeSocket)
-		tun.routeSocket = -1
-	} else if tun.events != nil {
-		close(tun.events)
-	}
+	var err1, err2 error
+    tun.closeOnce.Do(func () {
+        err1 = tun.tunFile.Close()
+        if tun.routeSocket != -1 {
+            unix.Shutdown(tun.routeSocket, unix.SHUT_RDWR)
+            err2 = unix.Close(tun.routeSocket)
+            tun.routeSocket = -1
+        } else if tun.events != nil {
+            close(tun.events)
+        }
+    })
 	if err1 != nil {
 		return err1
 	}