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>

conn/bindtest/bindtest.go

@@ -89,32 +89,39 @@ func (c *ChannelBind) Close() error {
 	return nil
 }
 
+func (c *ChannelBind) BatchSize() int { return 1 }
+
 func (c *ChannelBind) SetMark(mark uint32) error { return nil }
 
 func (c *ChannelBind) makeReceiveFunc(ch chan []byte) conn.ReceiveFunc {
-	return func(b []byte) (n int, ep conn.Endpoint, err error) {
+	return func(buffs [][]byte, sizes []int, eps []conn.Endpoint) (n int, err error) {
 		select {
 		case <-c.closeSignal:
-			return 0, nil, net.ErrClosed
+			return 0, net.ErrClosed
 		case rx := <-ch:
-			return copy(b, rx), c.target6, nil
+			copied := copy(buffs[0], rx)
+			sizes[0] = copied
+			eps[0] = c.target6
+			return 1, nil
 		}
 	}
 }
 
-func (c *ChannelBind) Send(b []byte, ep conn.Endpoint) error {
-	select {
-	case <-c.closeSignal:
-		return net.ErrClosed
-	default:
-		bc := make([]byte, len(b))
-		copy(bc, b)
-		if ep.(ChannelEndpoint) == c.target4 {
-			*c.tx4 <- bc
-		} else if ep.(ChannelEndpoint) == c.target6 {
-			*c.tx6 <- bc
-		} else {
-			return os.ErrInvalid
+func (c *ChannelBind) Send(buffs [][]byte, ep conn.Endpoint) error {
+	for _, b := range buffs {
+		select {
+		case <-c.closeSignal:
+			return net.ErrClosed
+		default:
+			bc := make([]byte, len(b))
+			copy(bc, b)
+			if ep.(ChannelEndpoint) == c.target4 {
+				*c.tx4 <- bc
+			} else if ep.(ChannelEndpoint) == c.target6 {
+				*c.tx6 <- bc
+			} else {
+				return os.ErrInvalid
+			}
 		}
 	}
 	return nil