title: Jenkins user is gone after macOS update
body: <p>I'm running Jenkins as a CI server on a Mac. It was running fine on macOS 10.12 with the typical setup with user jenkins. </p>
  
  <p>Today I upgraded macOS to 10.13 (High Sierra). Jenkins could not start after the upgrade process completed. Furthermore, there was no user jenkins on the system. All Jenkins files are there, but there is no jenkins user in Settings -> Users &amp; Groups. If I try to use jenkins user in Terminal, for instance if I try to change file ownership to jenkins with chown, I get:</p>
  
  <pre><code>chown: jenkins: illegal user name
  </code></pre>
  
  <p>How do I fix this?</p>