sysusers.d config files allow a package to use declarative
configuration instead of manually written maintainer scripts. This also
allows image-based systems to be created with /usr/ only, and also
allows for factory resetting a system and recreating /etc/ on boot.

https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html

Note that this does not require a hard dependency on systemd,
debhelper generates depetencies correctly so that they work out
of the box on non-systemd and non-linux Debian builds seamlessly.

  * The internal webserver now runs as a dedicated system user _lacme-www
    (and group nogroup) instead of www-data:www-data.  This is configurable
    in the [webserver] section of the lacme(8) configuration file.
  * The internal ACME client now runs as a dedicated system user _lacme-client
    (and group nogroup) instead of nobody:nogroup.  This is configurable in
    the [client] section of the lacme(8) configuration file.
  * The _lacme-www and _lacme-client system users are created automatically by
    lacme.postinst (hence a new Depends: adduser), and deleted on purge.  (So
    make sure not to chown any file to these internal users.)