multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

chanbackup/single.go

@@ -6,10 +6,10 @@ import (
 	"io"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -160,7 +160,7 @@ func NewSingle(channel *channeldb.OpenChannel,
 		// the backups plaintext don't carry any private information. When
 		// we go to recover, we'll present this in order to derive the
 		// private key.
-		_, shaChainPoint := btcec.PrivKeyFromBytes(btcec.S256(), b.Bytes())
+		_, shaChainPoint := btcec.PrivKeyFromBytes(b.Bytes())
 
 		shaChainRootDesc = keychain.KeyDescriptor{
 			PubKey: shaChainPoint,
@@ -369,13 +369,11 @@ func readRemoteKeyDesc(r io.Reader) (keychain.KeyDescriptor, error) {
 		return keychain.KeyDescriptor{}, err
 	}
 
-	keyDesc.PubKey, err = btcec.ParsePubKey(pub[:], btcec.S256())
+	keyDesc.PubKey, err = btcec.ParsePubKey(pub[:])
 	if err != nil {
 		return keychain.KeyDescriptor{}, err
 	}
 
-	keyDesc.PubKey.Curve = nil
-
 	return keyDesc, nil
 }
 
@@ -480,7 +478,7 @@ func (s *Single) Deserialize(r io.Reader) error {
 	// been specified or not.
 	if !bytes.Equal(shaChainPub[:], zeroPub[:]) {
 		s.ShaChainRootDesc.PubKey, err = btcec.ParsePubKey(
-			shaChainPub[:], btcec.S256(),
+			shaChainPub[:],
 		)
 		if err != nil {
 			return err