The NMKeepAlive instance is useful to find out when we should disconnect.
The moment when we start disconnecting, we don't care about it anymore.

Add a nm_keep_alive_disarm() function to suppress property change events about
the alive state, after that point. Emitting further events from that point
on is only confusing.

Yes, this means, a NMKeepAlive instance shall not be re-used for
multiple purposes. Create a separate keep-alive instace for each target
that should be guarded.

Also, once disarmed, we can release all resources that the NMKeepAlive instance
was holding until now.