When using a private connection, we need to use dbus_g_proxy_new_for_peer()
because the bus isn't involved. Since many parts of libnm-glib create a
proxy for their corresponding remote object, consolidate the proxy creation
logic.
A later patch will add logic to use a private connection versus a bus-based
one.
Do NMSettingConnection:interface-name matching on the client side as
well, so that, eg, nm-applet does not list connections under the wrong
device.
(Also, move some return-if-fail checks from the subclass method
implementations into the wrapper function.)
https://bugzilla.gnome.org/show_bug.cgi?id=693684
