sd-bus: make introspection data non-recursive

sd-bus: make introspection data non-recursive

Currently, our introspection data looks like this:

        <node>
         <interface name="org.freedesktop.DBus.Peer">
           ...
         </interface>
         <interface name="org.freedesktop.DBus.Introspectable">
           ...
         </interface>
         <interface name="org.freedesktop.DBus.Properties">
           ...
         </interface>
         <node name="org"/>
         <node name="org/freedesktop"/>
         <node name="org/freedesktop/login1"/>
         <node name="org/freedesktop/login1/user"/>
         <node name="org/freedesktop/login1/user/self"/>
         <node name="org/freedesktop/login1/user/_1000"/>
         <node name="org/freedesktop/login1/seat"/>
         <node name="org/freedesktop/login1/seat/self"/>
         <node name="org/freedesktop/login1/seat/seat0"/>
         <node name="org/freedesktop/login1/session"/>
         <node name="org/freedesktop/login1/session/self"/>
         <node name="org/freedesktop/login1/session/c1"/>
        </node>

(ordered alphabetically for better visibility)

This is grossly incorrect. The spec says that we're allowed to return
non-directed children, however, it does not allow us to return data
recursively in multiple parents. If we return "org", then we must not
return anything else that starts with "org/".

It is unclear, whether we can include child-nodes as a tree. Moreover, it
is usually not what the caller wants. Hence, this patch changes sd-bus to
never return introspection data recursively. Instead, only a single
child-layer is returned.

This patch relies on enumerators to never return hierarchies. If someone
registers an enumerator via sd_bus_add_enumerator, they better register
sub-enumerators if they support *TRUE* hierarchies. Each enumerator is
treated as a single layer and not filtered.
Enumerators are still allowed to return nested data. However, that data
is still required to be a single hierarchy. For instance, returning
"/org/foo" and "/com/bar" is fine, but including "/com" or "/org" in that
dataset is not.
This should be the default for enumerators and I see no reason to filter
in sd-bus. Moreover, filtering that data-set would require to sort the
strv by path and then do prefix-filtering. This is O(n log n), which
would be fine, but still better to avoid.

Fixes #664.