NMIfupdownConnection really is a subclass of NMSysconfigConnection (as
declared via the G_DEFINE_TYPE macro in nm-ifconfig-connection.c), but
the header incorrectly used NMExportedConnection* in the class and
instance structs. We got away with it because NMSysconfigConnection*
didn't contain anything other than the stuff inherited from
NMExportedConnection*, but it would have caused much trouble if we did
add something.