[$] User=0day considered harmful in systemd

<p>Validating user input is a long-established security best practice, butthere can be differences of opinion about what should be done when thatvalidation fails. A recently reported bug in systemd has fostered adiscussion on that topic; along the way there has also been discussionabout how much validation systemd should actually be doing and how much should be left upto the underlying distribution. The controversy all revolves aroundusernames that systemd does not accept, but that some distributions (andPOSIX) find to be perfectly acceptable.