sd-event: define a new PREPARING state

sd-event: define a new PREPARING state

We already have a state RUNNING and EXITING when we dispatch regular and
exit callbacks. Let's introduce a new state called PREPARING that is
active while we invoke preparation callbacks. This way we have a state
each for all three kinds of event handlers.

The states are currently not documented, hence let's add a new state to
the end, before we start documenting this.