Incorrect max handshake message payload size calculation

As stated in spec,
         
> A Noise **handshake message** is also less than or equal to 65535 bytes.

It includes plain & encrypted public keys as well as authentication tag, provided by AEAD cipher.

So, this code
```go
	if len(payload) > MaxMsgLen {
		panic("noise: message is too long")
	}
```

is incorrect. MaxMsgLen depends on DHLen & token/pattern
