core: fix require $XDG_RUNTIME_DIR

  $ sudo dracut -f
  Trying to run as user instance, but $XDG_RUNTIME_DIR is not set.
  /usr/lib/dracut/modules.d/98systemd/module-setup.sh: line 10:
    ((: >= 198 : syntax error: operand expected (error token is ">= 198 ")
bus: allow reading empty arrays
bus: fix bus_message_read_strv

message_read_strv_extend returns 0 on success
bus: fix bus_print_property with strv
bus: rename sd_bus_get_property_{trivial,basic}
bus: mark sd_bus_message_at_end public
analyze: split out bus util functions

They will soon be used in systemctl
systemctl: port to libsystemd-bus