mount: add --devname to set the device name sent to FUSE for mount display

Before this change, the device name was always the remote:path rclone
was configured with. However this can contain sensitive information
and it appears in the `mount` output, so `--devname` allows the user
to configure it.

See: https://forum.rclone.org/t/rclone-mount-blomp-problem/29151/11

cmd/mount/mount.go

@@ -86,7 +86,7 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (<-chan error
 
 	f := VFS.Fs()
 	fs.Debugf(f, "Mounting on %q", mountpoint)
-	c, err := fuse.Mount(mountpoint, mountOptions(VFS, f.Name()+":"+f.Root(), opt)...)
+	c, err := fuse.Mount(mountpoint, mountOptions(VFS, opt.DeviceName, opt)...)
 	if err != nil {
 		return nil, nil, err
 	}