rpc: modify SignMessage+VerifyMessage to bind msg to LN context

In this commit, we modify the SignMessage+VerifyMessage slightly in
order to bind the message we're signing to our specific context. We do
this by pre-pending the ascii text "Lightning Signed Message:" to the
msg before we run it through dsha256. Adding this prefix binds the
message to our context in order to avoid accidentally signing a sighash
or the like.

nodesigner.go

@@ -67,8 +67,9 @@ func (n *nodeSigner) SignDigestCompact(hash []byte) ([]byte, error) {
 	isCompressedKey := true
 
 	// btcec.SignCompact returns a pubkey-recoverable signature
-	sig, err := btcec.SignCompact(btcec.S256(), n.privKey, hash,
-		isCompressedKey)
+	sig, err := btcec.SignCompact(
+		btcec.S256(), n.privKey, hash, isCompressedKey,
+	)
 	if err != nil {
 		return nil, fmt.Errorf("can't sign the hash: %v", err)
 	}