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/webdav/webdav.go

@@ -96,15 +96,17 @@ func init() {
 				Help:  "Other site/service or software",
 			}},
 		}, {
-			Name: "user",
-			Help: "User name.\n\nIn case NTLM authentication is used, the username should be in the format 'Domain\\User'.",
+			Name:      "user",
+			Help:      "User name.\n\nIn case NTLM authentication is used, the username should be in the format 'Domain\\User'.",
+			Sensitive: true,
 		}, {
 			Name:       "pass",
 			Help:       "Password.",
 			IsPassword: true,
 		}, {
-			Name: "bearer_token",
-			Help: "Bearer token instead of user/pass (e.g. a Macaroon).",
+			Name:      "bearer_token",
+			Help:      "Bearer token instead of user/pass (e.g. a Macaroon).",
+			Sensitive: true,
 		}, {
 			Name:     "bearer_token_command",
 			Help:     "Command to run to get a bearer token.",