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/pcloud/pcloud.go

@@ -110,10 +110,11 @@ func init() {
 				encoder.EncodeBackSlash |
 				encoder.EncodeInvalidUtf8),
 		}, {
-			Name:     "root_folder_id",
-			Help:     "Fill in for rclone to use a non root folder as its starting point.",
-			Default:  "d0",
-			Advanced: true,
+			Name:      "root_folder_id",
+			Help:      "Fill in for rclone to use a non root folder as its starting point.",
+			Default:   "d0",
+			Advanced:  true,
+			Sensitive: true,
 		}, {
 			Name: "hostname",
 			Help: `Hostname to connect to.
@@ -138,7 +139,8 @@ with rclone authorize.
 This is only required when you want to use the cleanup command. Due to a bug
 in the pcloud API the required API does not support OAuth authentication so
 we have to rely on user password authentication for it.`,
-			Advanced: true,
+			Advanced:  true,
+			Sensitive: true,
 		}, {
 			Name:       "password",
 			Help:       "Your pcloud password.",