- since commit d17d2688, a
  NMSettingsConnection no longer "is-a" NMConnection. Instead,
  we must call nm_settings_connection_get_connection() to obtain
  the NMConnection instance. Adjust this in mirror_8021x_connection()

- don't leak "ssid" in mirror_8021x_connection()

- move deletion of the mirror-connection to known_network_data_free().
  Previously, we must have made sure that every g_hash_table_remove()
  and g_hash_table_insert()(!!) first deletes the mirror connection.
  Likewise, in got_object_manager() when we call g_hash_table_remove_all(),
  delete created mirror connections.

- rework interface_added() to make it robust against calling
  interface_added() more than once without removing the interface
  in between. Essentially, this just means that we first look into
  "priv->known_networks" to see whether the @id is already tracked.
  And if so, delete an existing mirror-connection as necessary.