This adds a "sysusers.d" snippet for creating the system user "dbus" at
boot, if it is missing, in order to support stateless systems that boot
up with an empty /etc and need static information for determining which
system users to create.
This is only installed on systemd-based systems.
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99162
