bus: add sd_bus_emit_object_{added/removed}()

This implements two new helpers, discussed on systemd-devel about 1 year
ago:
    sd_bus_emit_object_added()
    sd_bus_emit_object_removed()

Both calls are equivalent to their respective counterpart
sd_bus_emit_interfaces_{added/removed}(), but can figure out the list of
interfaces themselves, instead of requiring the caller to provide them.
Furthermore, both calls properly deal with builtin interfaces provided via
org.freedesktop.DBus.* and alike.

Both calls simply traverse a node and all its parent nodes to figure out a
list of all interfaces registered as vtable or fallback. It then appends
each of them, similar to the interfaces_{added/removed}() helpers.

Note that interfaces_{added/removed}() runs a parent traversal for *each*
passed interface. Therefore, it can simply bail out, once it found a
parent node that implements a given interface.
With object_{added/removed}() we cannot know the registered interfaces in
advance, thus, we cannot run one traversal per node. Instead, we run a
single traversal and remember all interfaces that we added. Therefore, a
child-interface overrides all conflicting parent-interfaces. We keep a
"Set *s" context to track those while climbing up the tree.