dropbox: add option to override root namespace

This lets you, for example, use shared folders without mounting them
into your home namespace first, as long as you know their namespace ID.

(The --dropbox-shared-folders flag could thus be changed to not need to
mount the shared folder first, but I'm not doing that here as it's a
behavior change, who knows, maybe somebody relies on it.)

backend/dropbox/dropbox.go

@@ -216,7 +216,10 @@ are supported.
 
 Note that we don't unmount the shared folder afterwards so the 
 --dropbox-shared-folders can be omitted after the first use of a particular 
-shared folder.`,
+shared folder.
+
+See also --dropbox-root-namespace for an alternative way to work with shared
+folders.`,
 			Default:  false,
 			Advanced: true,
 		}, {
@@ -237,6 +240,11 @@ shared folder.`,
 				encoder.EncodeDel |
 				encoder.EncodeRightSpace |
 				encoder.EncodeInvalidUtf8,
+		}, {
+			Name:     "root_namespace",
+			Help:     "Specify a different Dropbox namespace ID to use as the root for all paths.",
+			Default:  "",
+			Advanced: true,
 		}}...), defaultBatcherOptions.FsOptions("For full info see [the main docs](https://rclone.org/dropbox/#batch-mode)\n\n")...),
 	})
 }
@@ -253,6 +261,7 @@ type Options struct {
 	AsyncBatch    bool                 `config:"async_batch"`
 	PacerMinSleep fs.Duration          `config:"pacer_min_sleep"`
 	Enc           encoder.MultiEncoder `config:"encoding"`
+	RootNsid      string               `config:"root_namespace"`
 }
 
 // Fs represents a remote dropbox server
@@ -502,8 +511,11 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 
 	f.features.Fill(ctx, f)
 
-	// If root starts with / then use the actual root
+	if f.opt.RootNsid != "" {
-	if strings.HasPrefix(root, "/") {
+		f.ns = f.opt.RootNsid
+		fs.Debugf(f, "Overriding root namespace to %q", f.ns)
+	} else if strings.HasPrefix(root, "/") {
+		// If root starts with / then use the actual root
 		var acc *users.FullAccount
 		err = f.pacer.Call(func() (bool, error) {
 			acc, err = f.users.GetCurrentAccount()