Rename user_runtime to user_runtime_dir

This makes this function name similar to user_config_home() and makes
it match the name of the environment variable.
Fix order and document user unit dirs

Fixup for 718880ba0d 'add a transient user unit directory'.
virt: detect that we are running inside the docker container
sd-bus: sync kdbus.h (ABI break!)

struct kdbus_cmd_match got a flags field, which systemd currently makes no
use of.