Extend fs_mistrust with APIs for file removal and listing

Most Arti keystore operations fail if the keystore permissions are not restrictive enough. However, since list() and remove() are not implemented using fs_mistrust APIs, they both fail to return an error when the permissions are too lax. We probably want list() and remove() to have the same behavior as the other keystore operations (i.e. fail if the keystore permissions are not acceptable).