I found the handling of the master-device very confusing because it was
unclear who sets priv->master, and when it should be set.

Now:

- Setting priv->master (in a slave) always goes together with adding
  the master to priv->slaves (in the master). Previously, this was
  done at separate places, so it was not clear if master and slave
  always agree on their relationship -- in fact, they did not.

- There are now three basic functions which do the enslaving/releasing:
    (1) nm_device_master_add_slave()
    (2) nm_device_master_enslave_slave()
    (3) nm_device_master_release_one_slave()
  Step 3/release basically undoes the 1/add and 2/enslave steps.

- completing the enslaving/releasing is now done by
    (1) nm_device_slave_notify_enslave()
    (2) nm_device_slave_notify_release()
  These functions also emit signals like NM_DEVICE_MASTER.

- Derived classes no longer emit NM_DEVICE_SLAVES notification. Instead
  the notification is emited together with NM_DEVICE_MASTER, whenever a
  slaves changes state. Also, NM_DEVICE_SLAVES list now only exposes
  slaves that are actually @is_enslaved.