Make it possible to construct the plugin instance in a way that disconnects the
connection if the DBus client that activated it drops off the bus. This makes the
plugins conveniently clean up when NetworkManager crashes.

We need this, as with multiple VPN support we can loose track of the client bus
names when the daemon crashes leaving to nice way to clean up on respawn.

However, this behavior is not desired for debugging or hypotetical VPN plugin
users other than NetworkManager (say; "gdbus call -m o.fd.NM.VPN.Plugin.Connect").
Let the plugin decide when to use it.