conn, device, tun: implement vectorized I/O plumbing

Accept packet vectors for reading and writing in the tun.Device and
conn.Bind interfaces, so that the internal plumbing between these
interfaces now passes a vector of packets. Vectors move untouched
between these interfaces, i.e. if 128 packets are received from
conn.Bind.Read(), 128 packets are passed to tun.Device.Write(). There is
no internal buffering.

Currently, existing implementations are only adjusted to have vectors
of length one. Subsequent patches will improve that.

Also, as a related fixup, use the unix and windows packages rather than
the syscall package when possible.

Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: James Tucker <james@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/tuntest/tuntest.go

@@ -110,35 +110,42 @@ type chTun struct {
 
 func (t *chTun) File() *os.File { return nil }
 
-func (t *chTun) Read(data []byte, offset int) (int, error) {
+func (t *chTun) Read(packets [][]byte, sizes []int, offset int) (int, error) {
 	select {
 	case <-t.c.closed:
 		return 0, os.ErrClosed
 	case msg := <-t.c.Outbound:
-		return copy(data[offset:], msg), nil
+		n := copy(packets[0][offset:], msg)
+		sizes[0] = n
+		return 1, nil
 	}
 }
 
 // Write is called by the wireguard device to deliver a packet for routing.
-func (t *chTun) Write(data []byte, offset int) (int, error) {
+func (t *chTun) Write(packets [][]byte, offset int) (int, error) {
 	if offset == -1 {
 		close(t.c.closed)
 		close(t.c.events)
 		return 0, io.EOF
 	}
-	msg := make([]byte, len(data)-offset)
-	copy(msg, data[offset:])
-	select {
-	case <-t.c.closed:
-		return 0, os.ErrClosed
-	case t.c.Inbound <- msg:
-		return len(data) - offset, nil
+	for i, data := range packets {
+		msg := make([]byte, len(data)-offset)
+		copy(msg, data[offset:])
+		select {
+		case <-t.c.closed:
+			return i, os.ErrClosed
+		case t.c.Inbound <- msg:
+		}
 	}
+	return len(packets), nil
+}
+
+func (t *chTun) BatchSize() int {
+	return 1
 }
 
 const DefaultMTU = 1420
 
-func (t *chTun) Flush() error             { return nil }
 func (t *chTun) MTU() (int, error)        { return DefaultMTU, nil }
 func (t *chTun) Name() (string, error)    { return "loopbackTun1", nil }
 func (t *chTun) Events() <-chan tun.Event { return t.c.events }