Maximum segment size fixes (#25)

* Fixed MSS sent in SYN and SYN-ACK

* Fixed tests

x/xnet/xnet_concurrent_test.go

@@ -140,7 +140,8 @@ func TestTCPListener_ConcurrentEcho(t *testing.T) {
 
 func kernelLoop(ctx context.Context, server *StackAsync, clients []StackAsync) {
 	const MTU = 1500
-	buf := make([]byte, MTU)
+	const carrierDataSize = MTU + 14
+	buf := make([]byte, carrierDataSize)
 	rng := rand.New(rand.NewSource(1)) // Seed 1 for deterministic but randomized order
 	order := make([]int, len(clients))
 	for i := range order {

x/xnet/xnet_dns_test.go

@@ -47,7 +47,8 @@ func TestDNS_QueryReceivesAnswer(t *testing.T) {
 	}
 
 	// Client sends DNS query.
-	var buf [MTU]byte
+	const carrierDataSize = MTU + 14
+	var buf [carrierDataSize]byte
 	n, err := client.Encapsulate(buf[:], -1, 0)
 	if err != nil {
 		t.Fatal("client Encapsulate failed:", err)

x/xnet/xnet_test.go

@@ -217,9 +217,10 @@ func newTCPStacks(t *testing.T, randSeed int64, mtu int) (s1, s2 *StackAsync, c1
 }
 
 func testerFrom(t *testing.T, mtu int) *tester {
+	carrierDataSize := mtu + 14
 	return &tester{
 		t:   t,
-		buf: make([]byte, mtu),
+		buf: make([]byte, carrierDataSize),
 	}
 }