When --db and --dumpDbUsersAndRoles are both specified, mongodump will dump out special files for the users and roles of a single db. These files are prefixed with $ so they aren't mistaken for actual collections. We need to change the special handling in mongorestore to specifically check for three filenames so that they don't get confused for the long collection name filenames that also include %24 ($):

$admin.system.users
$admin.system.roles
$admin.system.version