sd-bus: sync kdbus.h

kdbus learned a new command to query a bus creator's credentials. Sync
kdbus.h first, which also renames some struct to more generic terms.
That is, however, not an ABI break this time.
sd-bus: factor out creds item iterator

We will re-use the code to walk items in order to populate a creds object,
so let's factor it out first.
sd-bus: rename sd_bus_get_owner_uid(), sd_bus_get_owner_machine_id() and sd_bus_get_peer_creds()

Clean up the function namespace by renaming the following:

  sd_bus_get_owner_uid()        → sd_bus_get_name_creds_uid()
  sd_bus_get_owner_machine_id() → sd_bus_get_name_machine_id()
  sd_bus_get_peer_creds()       → sd_bus_get_owner_creds()
sd-bus: implement sd_bus_get_owner_creds() for kdbus

kdbus learned a new ioctl to tell userspace about a bus creator's
credentials, which is what we need to implement sd_bus_get_owner_creds() for
kdbus.

Move the function from sd-bus.c to bus-control.c to be able to reuse
the bus_populate_creds_from_items() helper.