- when a caller sets the is-default flag on a CA, and another CA is no
      longer the default, emit the PropertiesChanged signal on the CA which is
      not the default, instead on the new default a second time
    - drop some dead code from the D-Bus message handlers (static analysis, #796813)
    - cache public keys when we read private keys
    - go back to printing an error indicating that we're missing a required
      argument when we're missing a required argument, not that the option is
      invalid (broken since 0.51, #796542)