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

cert/selfsigned.go

@@ -194,7 +194,7 @@ func IsOutdated(cert *x509.Certificate, tlsExtraIPs,
 // real PKI.
 //
 // This function is adapted from https://github.com/btcsuite/btcd and
-// https://github.com/btcsuite/btcutil
+// https://github.com/btcsuite/btcd/btcutil
 func GenCertPair(org, certFile, keyFile string, tlsExtraIPs,
 	tlsExtraDomains []string, tlsDisableAutofill bool,
 	certValidity time.Duration) error {