config: determine the bitcoind chain directory from the chain params instead of the config file. This is more reliable in case the chain was selected with a bitcoind commandline option instead of a line in the config file.

config.go

@@ -1046,19 +1046,14 @@ func extractBitcoindRPCParams(bitcoindConfigPath string) (string, string, string
 	}
 
 	chainDir := "/"
-	netRE, err := regexp.Compile(`(?m)^\s*(testnet|regtest)=[\d]+`)
+	switch activeNetParams.Params.Name {
-	if err != nil {
+	case "testnet3":
-		return "", "", "", err
-	}
-	netSubmatches := netRE.FindSubmatch(configContents)
-	if netSubmatches != nil {
-		switch string(netSubmatches[1]) {
-		case "testnet":
 		chainDir = "/testnet3/"
+	case "testnet4":
+		chainDir = "/testnet4/"
 	case "regtest":
 		chainDir = "/regtest/"
 	}
-	}
 
 	cookie, err := ioutil.ReadFile(dataDir + chainDir + ".cookie")
 	if err == nil {