p2p: msg.Payload contains list data

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

p2p/peer.go

@@ -193,12 +193,12 @@ func (p *Peer) handle(msg Msg) error {
 		msg.Discard()
 		go EncodeMsg(p.rw, pongMsg)
 	case msg.Code == discMsg:
-		var reason DiscReason
+		var reason [1]DiscReason
 		// no need to discard or for error checking, we'll close the
 		// connection after this.
 		rlp.Decode(msg.Payload, &reason)
 		p.Disconnect(DiscRequested)
-		return discRequestedError(reason)
+		return discRequestedError(reason[0])
 	case msg.Code < baseProtocolLength:
 		// ignore other base protocol messages
 		return msg.Discard()