On systems without properly setup systemd, cg_get_root_path returns
-ENOENT. This means that busctl doesn't display much information.
busctl monitor also fails whenever it intercepts messages.
This fix fakes creates a fake "/" root cgroup which lets busctl work
on such systems.
These are the counterpart of "floating" bus slots, i.e. event sources
that are bound to the lifetime of the event object itself, and thus
don't require an explicit reference to be kept.
This makes callback behaviour more like sd-event or sd-resolve, and
creates proper object for unregistering callbacks.
Taking the refernce to the slot is optional. If not taken life time of
the slot will be bound to the underlying bus object (or in the case of
an async call until the reply has been recieved).
This is primarily useful for services that need to track clients which
reference certain objects they maintain, or which explicitly want to
subscribe to certain events. Something like this is done in a large
number of services, and not trivial to do. Hence, let's unify this at
one place.
This also ports over PID 1 to use this to ensure that subscriptions to
job and manager events are correctly tracked. As a side-effect this
makes sure we properly serialize and restore the track list across
daemon reexec/reload, which didn't work correctly before.
This also simplifies how we distribute messages to broadcast to the
direct busses: we only track subscriptions for the API bus and
implicitly assume that all direct busses are subscribed. This should be
a pretty OK simplification since clients connected via direct bus
connections are shortlived anyway.
sizes and numbers of hash functions
In order to make the bloom filter logic more future proof communicate
bloom filter parameters from the original bus creator to the clients,
and allow them to be variable within certain ranges.
