terminal+config: implement remote lnd mode

With this commit we add all config options that are necessary to connect
to a remote lnd node instead of starting one in the same process as LiT.

Author: guggero

cmd/litd/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"os"
 
@@ -11,8 +12,9 @@ import (
 // main starts the lightning-terminal application.
 func main() {
 	err := terminal.New().Run()
-	if e, ok := err.(*flags.Error); err != nil &&
-		(!ok || e.Type != flags.ErrHelp) {
+	var flagErr *flags.Error
+	if err != nil && errors.As(err, &flagErr) &&
+		flagErr.Type != flags.ErrHelp {
 
 		_, _ = fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)