manager: explicitly create our private runtime directory

manager: explicitly create our private runtime directory

So far /run/systemd/ was created as side-effect of initializing the
D-Bus client/server. But in one of the next commits we'll suppress
connecting to D-Bus in test runs, hence let's move the logic our of the
D-Bus code and into manager_startup().

Then, also drop creating it again and again in PID 1 at various places,
and just rely on it to exist.