rtkit is a dbus service needed for pipewire to apply for a realtime
priority.

It needs a separate user to run.

  groupadd -g 378 rtkit
  useradd -u 378 -s /bin/false -d /var/lib/rtkit \
    -g rtkit rtkit

In principle, pipewire may function without this service, but it
helps to placate some error messages present on Slackware 15.0
default setup.
