rclone config redacted: implement support mechanism for showing redacted config

This introduces a new fs.Option flag, Sensitive and uses this along
with IsPassword to redact the info in the config file for support
purposes.

It adds this flag into backends where appropriate. It was necessary to
add oauthutil.SharedOptions to some backends as they were missing
them.

Fixes #5209

backend/seafile/seafile.go

@@ -67,15 +67,18 @@ func init() {
 				Value: "https://cloud.seafile.com/",
 				Help:  "Connect to cloud.seafile.com.",
 			}},
+			Sensitive: true,
 		}, {
-			Name:     configUser,
-			Help:     "User name (usually email address).",
-			Required: true,
+			Name:      configUser,
+			Help:      "User name (usually email address).",
+			Required:  true,
+			Sensitive: true,
 		}, {
 			// Password is not required, it will be left blank for 2FA
 			Name:       configPassword,
 			Help:       "Password.",
 			IsPassword: true,
+			Sensitive:  true,
 		}, {
 			Name:    config2FA,
 			Help:    "Two-factor authentication ('true' if the account has 2FA enabled).",
@@ -87,6 +90,7 @@ func init() {
 			Name:       configLibraryKey,
 			Help:       "Library password (for encrypted libraries only).\n\nLeave blank if you pass it through the command line.",
 			IsPassword: true,
+			Sensitive:  true,
 		}, {
 			Name:     configCreateLibrary,
 			Help:     "Should rclone create a library if it doesn't exist.",
@@ -94,9 +98,10 @@ func init() {
 			Default:  false,
 		}, {
 			// Keep the authentication token after entering the 2FA code
-			Name: configAuthToken,
-			Help: "Authentication token.",
-			Hide: fs.OptionHideBoth,
+			Name:      configAuthToken,
+			Help:      "Authentication token.",
+			Hide:      fs.OptionHideBoth,
+			Sensitive: true,
 		}, {
 			Name:     config.ConfigEncoding,
 			Help:     config.ConfigEncodingHelp,