sd-bus: rework credential query logic

Also, make the call to free kdbus slices generic and use it everywhere
sd-bus: add new call sd_bus_get_scope() for querying whether one is connected to a system or a user bus
sd-bus: rename default bus address constants, they aren't "paths" but "addresses"
bus-proxy: automatically detect scope of bus and derive which XML snippets to load from that
update TODO