cmd: Migrate to spf13/cobra, remove single-dash arg support (#4565)

* cmd: migrate to spf13/cobra

* add `manpage` command

* limit Caddy tagline to root `help` only

* hard-code the manpage section to 8

cmd/main.go

@@ -33,6 +33,7 @@ import (
 	"github.com/caddyserver/caddy/v2"
 	"github.com/caddyserver/caddy/v2/caddyconfig"
 	"github.com/caddyserver/certmagic"
+	"github.com/spf13/pflag"
 	"go.uber.org/zap"
 )
 
@@ -58,35 +59,9 @@ func Main() {
 		os.Args = append(os.Args, "help")
 	}
 
-	subcommandName := os.Args[1]
-	subcommand, ok := commands[subcommandName]
-	if !ok {
-		if strings.HasPrefix(os.Args[1], "-") {
-			// user probably forgot to type the subcommand
-			fmt.Println("[ERROR] first argument must be a subcommand; see 'caddy help'")
-		} else {
-			fmt.Printf("[ERROR] '%s' is not a recognized subcommand; see 'caddy help'\n", os.Args[1])
-		}
-		os.Exit(caddy.ExitCodeFailedStartup)
+	if err := rootCmd.Execute(); err != nil {
+		os.Exit(1)
 	}
-
-	fs := subcommand.Flags
-	if fs == nil {
-		fs = flag.NewFlagSet(subcommand.Name, flag.ExitOnError)
-	}
-
-	err := fs.Parse(os.Args[2:])
-	if err != nil {
-		fmt.Println(err)
-		os.Exit(caddy.ExitCodeFailedStartup)
-	}
-
-	exitCode, err := subcommand.Func(Flags{fs})
-	if err != nil {
-		fmt.Fprintf(os.Stderr, "%s: %v\n", subcommand.Name, err)
-	}
-
-	os.Exit(exitCode)
 }
 
 // handlePingbackConn reads from conn and ensures it matches
@@ -280,7 +255,7 @@ func watchConfigFile(filename, adapterName string) {
 // Flags wraps a FlagSet so that typed values
 // from flags can be easily retrieved.
 type Flags struct {
-	*flag.FlagSet
+	*pflag.FlagSet
 }
 
 // String returns the string representation of the
@@ -326,22 +301,6 @@ func (f Flags) Duration(name string) time.Duration {
 	return val
 }
 
-// flagHelp returns the help text for fs.
-func flagHelp(fs *flag.FlagSet) string {
-	if fs == nil {
-		return ""
-	}
-
-	// temporarily redirect output
-	out := fs.Output()
-	defer fs.SetOutput(out)
-
-	buf := new(bytes.Buffer)
-	fs.SetOutput(buf)
-	fs.PrintDefaults()
-	return buf.String()
-}
-
 func loadEnvFromFile(envFile string) error {
 	file, err := os.Open(envFile)
 	if err != nil {