config: Explicitly handle help requests.

Checking for --help as an explicit step before parsing any other configs
makes the code more intuitive by removing a convoluted bit of error
handling.

It also enables the IgnoreUnknown option to be used whilst parsing for
help, which ensures the presence of --help will always result in the
help message being printed. This fixes a minor inconsistency where the
help message would be printed if the flag was placed before an invalid
config, but placing it after would cause an invalid config error to be
written instead. For example, `dcrd --help --fakeflag` vs `dcrd
--fakeflag --help`.

Author: jholdstock

config.go

@@ -585,9 +585,10 @@ func (e errSuppressUsage) Error() string {
 //
 // The configuration proceeds as follows:
 //  1. Start with a default config with sane settings
-//  2. Pre-parse the command line to check for an alternative config file
-//  3. Load configuration file overwriting defaults with any specified options
-//  4. Parse CLI options and overwrite/add any specified options
+//  2. Check if help has been requested, print it and exit if so
+//  3. Pre-parse the command line to check for an alternative config file
+//  4. Load configuration file overwriting defaults with any specified options
+//  5. Parse CLI options and overwrite/add any specified options
 //
 // The above results in dcrd functioning properly without any config settings
 // while still allowing the user to override settings with config files and
@@ -648,23 +649,23 @@ func loadConfig(appName string) (*config, []string, error) {
 		params:       &mainNetParams,
 	}
 
+	// Pre-parse the command line options looking only for the help option. If
+	// found, print the help message to stdout and exit.
+	helpOpts := flags.Options(flags.HelpFlag | flags.PrintErrors | flags.IgnoreUnknown)
+	_, err := flags.NewParser(&cfg, helpOpts).Parse()
+	if flags.WroteHelp(err) {
+		os.Exit(0)
+	}
+
 	// Service options which are only added on Windows.
 	serviceOpts := serviceOptions{}
 
-	// Pre-parse the command line options to see if an alternative config
-	// file or the version flag was specified.  Any errors aside from the
-	// help message error can be ignored here since they will be caught by
-	// the final parse below.
+	// Pre-parse the command line options to see if an alternative config file
+	// or the version flag was specified.  Any errors can be ignored here since
+	// they will be caught by the final parse below.
 	preCfg := cfg
-	preParser := newConfigParser(&preCfg, &serviceOpts, flags.HelpFlag)
-	_, err := preParser.Parse()
-	if err != nil {
-		var e *flags.Error
-		if errors.As(err, &e) && e.Type == flags.ErrHelp {
-			fmt.Fprintln(os.Stdout, err)
-			os.Exit(0)
-		}
-	}
+	preParser := newConfigParser(&preCfg, &serviceOpts, flags.None)
+	_, _ = preParser.Parse()
 
 	// Show the version and exit if the version flag was specified.
 	if preCfg.ShowVersion {
@@ -739,7 +740,7 @@ func loadConfig(appName string) (*config, []string, error) {
 
 	// Load additional config from file.
 	var configFileError error
-	parser := newConfigParser(&cfg, &serviceOpts, flags.HelpFlag|flags.PassDoubleDash)
+	parser := newConfigParser(&cfg, &serviceOpts, flags.PassDoubleDash)
 	if !(cfg.SimNet || cfg.RegNet) || preCfg.ConfigFile != defaultConfigFile {
 		err := flags.NewIniParser(parser).ParseFile(preCfg.ConfigFile)
 		if err != nil {