lnd+cmd/lnd/main: add ListenerCfg to Main

ListenerCfg allows passing custom listeners to the main method, to be
used for the wallet unlocker and rpc server. If these are set these will
be used instead of the regular RPC listeners.

cmd/lnd/main.go

@@ -11,7 +11,7 @@ import (
 func main() {
 	// Call the "real" main in a nested manner so the defers will properly
 	// be executed in the case of a graceful shutdown.
-	if err := lnd.Main(); err != nil {
+	if err := lnd.Main(lnd.ListenerCfg{}); err != nil {
 		if e, ok := err.(*flags.Error); ok && e.Type == flags.ErrHelp {
 		} else {
 			fmt.Fprintln(os.Stderr, err)