Default to using both policy and /etc/brlapi.key

  * patches/defauth-keyfile.patch: Fix using both polkit and keyfile.
  * brltty.postinst: Do not make /etc/brlapi.key readable by world any more,
    polkit should be enough.
  * patches/defauth-polkit: Default to using both policykit and
    /etc/brlapi.key to authenticate brlapi clients.