title: systemd error with user having a dot (.) in name: Invalid user/group name or numeric ID
body: <p>Linux Ubuntu 18. MariaDB 10.3.
  MariaDB service fails to start throwing a systemd error. The reason seems to be that the userid contains a dot (.).</p>
  <p><code>systemd[1]: Invalid user/group name or numeric ID: myuser.withdot</code></p>
  <p>Configuration:</p>
  <p>The service file from MariaDB is /etc/systemd/system/mariadb.service.d/migrated.conf, and it contains:</p>
  <pre><code>[Service]
  User=myuser.withdot
  </code></pre>