Eventually we should do a coordinated shutdown when NetworkManager exits.
That is a large work, ensuring that all asynchronous actions are cancellable
(in time), and that we wait for all pending operations to complete.

Add a function nm_shutdown_register_watchdog(), so that objects can register
themselves, to block shutdown until they are destroyed. It's not yet used,
because actually iterating the mainloop during shutdown can only be done,
once the code is prepared to be ready for that. But we already need the
function, so that we can refactor individual parts of the code, in preparation
of using it in the future.