Use single cert/key for RPC by default.

Also, while here, set the default RPC cert/key in the initial config
struct so the default will show when displaying the usage.

config.go

@@ -36,6 +36,8 @@ var (
 	defaultDataDir     = filepath.Join(btcdHomeDir, defaultDataDirname)
 	defaultListener    = net.JoinHostPort("", netParams(defaultBtcnet).listenPort)
 	knownDbTypes       = btcdb.SupportedDBs()
+	defaultRPCKeyFile  = filepath.Join(btcdHomeDir, "rpc.key")
+	defaultRPCCertFile = filepath.Join(btcdHomeDir, "rpc.cert")
 )
 
 // config defines the configuration options for btcd.
@@ -180,6 +182,8 @@ func loadConfig() (*config, []string, error) {
 		ConfigFile:  defaultConfigFile,
 		DataDir:     defaultDataDir,
 		DbType:      defaultDbType,
+		RPCKey:      defaultRPCKeyFile,
+		RPCCert:     defaultRPCCertFile,
 	}
 
 	// Pre-parse the command line options to see if an alternative config
@@ -342,13 +346,6 @@ func loadConfig() (*config, []string, error) {
 		}
 	}
 
-	if cfg.RPCKey == "" {
-		cfg.RPCKey = filepath.Join(cfg.DataDir, "rpc.key")
-	}
-	if cfg.RPCCert == "" {
-		cfg.RPCCert = filepath.Join(cfg.DataDir, "rpc.cert")
-	}
-
 	// Add default port to all listener addresses if needed and remove
 	// duplicate addresses.
 	cfg.Listeners = normalizeAddresses(cfg.Listeners,