This adds an optional "recursive" counting mode to sd_bus_track. If enabled
adding the same name multiple times to an sd_bus_track object is counted
individually, so that it also has to be removed the same number of times before
it is gone again from the tracking object.

This functionality is useful for implementing local ref counted objects that
peers make take references on.

And while ware at it, also drop some references to kdbus, and stop claiming
sd-bus wasn't stable yet. Also order man page references in the main sd-bus man
page alphabetically.

This adds two (privileged) bus calls Ref() and Unref() to the Unit interface.
The two calls may be used by clients to pin a unit into memory, so that various
runtime properties aren't flushed out by the automatic GC. This is necessary
to permit clients to race-freely acquire runtime results (such as process exit
status/code or accumulated CPU time) on successful service termination.

Ref() and Unref() are fully recursive, hence act like the usual reference
counting concept in C. Taking a reference is a privileged operation, as this
allows pinning units into memory which consumes resources.

Transient units may also gain a reference at the time of creation, via the new
AddRef property (that is only defined for transient units at the time of
creation).

Let's make sure we can read the exit code/status properties exposed by PID 1
properly. Let's reuse the existing code for unsigned fields, as we just use it
to copy words around, and don't calculate it.

Instead of ignoring empty strings retrieved via the bus, treat them as NULL, as
it's customary in systemd.

…nknown type

Make sure we return proper errors for types not understood yet.

It is useful for clients to be able to read the last CPU usage counter value of
a unit even if the unit is already terminated. Hence, before destroying a
cgroup's cgroup cache the last CPU usage counter and return it if the cgroup is
gone.

Also, make major improvements to the an page in general.

…unction

When a bus connection is closed we dispatch all reply callbacks. Do so in a new
function if its own.

No behaviour changes.

…mes to them

In order to add a name to a bus tracking object we need to do some bus
operations: we need to check if the name already exists and add match for it.
Both are synchronous bus calls. While processing those we need to make sure
that the tracking object is not dispatched yet, as it might still be empty, but
is not going to be empty for very long.

hence, block dispatching by removing the object from the dispatch queue while
adding it, and readding it on error.

…racking objects immediately

If the server side kicks us from the bus, from our view no names are on the bus
anymore, hence let's make sure to dispatch all tracking objects immediately.

This tests in particular that disconnecting results in the tracking object's
handlers to be called.

Old libdbus has a feature that the process is terminated whenever the the bus
connection receives a disconnect. This is pretty useful on desktop apps (where
a disconnect indicates session termination), as well as on command line apps
(where we really shouldn't stay hanging in most cases if dbus daemon goes
down).

Add a similar feature to sd-bus, but make it opt-in rather than opt-out, like
it is on libdbus. Also, if the bus is attached to an event loop just exit the
event loop rather than the the whole process.