cmd/syncthing: Mostly replace urfave/cli command line parser with alecthomas/kong (#9166)

`syncthing cli` subcommand was using urfave/cli as the command parser.
This PR replace it with kong, which the main command uses.

Some help texts and error message format are changed. Other than that,
all the command usage and logic remains unchanged.

There's only one place which still uses urfave/cli, which is `syncthing
cli config`, because it uses some magic to dynamically build commands
from struct reflects. I used kong's `passthrough:""` tag to pass any
argument following `syncthing cli config` to urfave/cli parser.

This PR also fixes #9041

---------

Co-authored-by: Jakob Borg <jakob@kastelo.net>

cmd/syncthing/main.go

@@ -139,7 +139,7 @@ var entrypoint struct {
 	Serve    serveOptions `cmd:"" help:"Run Syncthing"`
 	Generate generate.CLI `cmd:"" help:"Generate key and config, then exit"`
 	Decrypt  decrypt.CLI  `cmd:"" help:"Decrypt or verify an encrypted folder"`
-	Cli      struct{}     `cmd:"" help:"Command line interface for Syncthing"`
+	Cli      cli.CLI      `cmd:"" help:"Command line interface for Syncthing"`
 }
 
 // serveOptions are the options for the `syncthing serve` command.
@@ -213,17 +213,6 @@ func defaultVars() kong.Vars {
 }
 
 func main() {
-	// The "cli" subcommand uses a different command line parser, and e.g. help
-	// gets mangled when integrating it as a subcommand -> detect it here at the
-	// beginning.
-	if len(os.Args) > 1 && os.Args[1] == "cli" {
-		if err := cli.Run(); err != nil {
-			fmt.Println(err)
-			os.Exit(1)
-		}
-		return
-	}
-
 	// First some massaging of the raw command line to fit the new model.
 	// Basically this means adding the default command at the front, and
 	// converting -options to --options.
@@ -249,7 +238,15 @@ func main() {
 
 	// Create a parser with an overridden help function to print our extra
 	// help info.
-	parser, err := kong.New(&entrypoint, kong.Help(helpHandler), defaultVars())
+	parser, err := kong.New(
+		&entrypoint,
+		kong.ConfigureHelp(kong.HelpOptions{
+			NoExpandSubcommands: true,
+			Compact:             true,
+		}),
+		kong.Help(helpHandler),
+		defaultVars(),
+	)
 	if err != nil {
 		log.Fatal(err)
 	}