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Using systemd we can automate creating users and directories. So
generate and install the configuration files.
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>

Small change in cmake/install_layout.cmake compared to original contributor
patch to also install SYSTEMD_SYSUSERS and SYSTEMD_TMPFILES directories. The
variables were being set, but the loop which defines the final install files
was not updated.