Previously, there were two functions nm_ppp_manager_stop_sync() and
nm_ppp_manager_stop_async().

However, stop-sync() would still kill the process asynchronously (with a
2 seconds timeout before sending SIGKILL).

On the other hand, stop-async() did pretty much the same thing as
sync-code, except also using the GAsyncResult.

Merge the two functions. Stopping the instance for the most part can be
done entirely synchrnous. The only thing that is asynchronous, is
to wait for the process to terminate. For that, add a new callback
argument to nm_ppp_manager_stop(). This replaces the GAsyncResult
pattern.

Also, always ensure that NetworkManager runs the mainloop at least as
long until the process really terminated. Currently we don't get that
right, and during shutdown we just stop iterating the mainloop. However,
fix this from point of view of NMPPPManager and register a wait-object,
that later will correctly delay shutdown.

Also, NMDeviceWwan cared to wait (asynchronously) until pppd really
terminated. Keep that functionality. nm_ppp_manager_stop() returns
a handle that can be used to cancel the asynchrounous request and invoke
the callback right away. However note, that even when cancelling the
request, the wait-object that prevents shutdown of NetworkManager is
kept around, so that we can be sure to properly clean up.