and use NMIPConfig type.

Still unused...

Fixes: 1b1b4bd91c29425c25e8e979cd77b7a67deb9bf5

nm_ppp_manager_stop() wants to ensure that the pppd process is really
gone. For that it uses nm_utils_kill_child_async() to first send
SIGTERM, and sending SIGKILL after a timeout.

Later, we want to fix shutdown of NetworkManager to iterate the mainloop
during shutdown, so that such operations are still handled. However, we
can only delay shutdown for a certain time. After a timeout (NM_SHUTDOWN_TIMEOUT_MS
plus NM_SHUTDOWN_TIMEOUT_MS_GRACE) we really have to give up and
terminate.

That means, the right amount of time between sending SIGTERM and SIGKILL
is exactly NM_SHUTDOWN_TIMEOUT_MS. Hopefully that is of course
sufficient in the first place. If not, send SIGKILL afterwards, and give
a bit more time (NM_SHUTDOWN_TIMEOUT_MS_GRACE) to reap the child.
And if all this time is still not enough, something is really odd and we
abort waiting, with a warning in the logfile.

Since we don't properly handle shutdown yet, the description above is
not really true. But with this patch, we fix it from point of view of
NMPPPManager.

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.

We usually structure our code in a (pseudo) object oriented way.
It makes sense to call the variable for the target object "self",
it is more familiar and usually done.