Disable Ctrl-Alt-Delete has been change documentation needs updating

[AJ]

In 16.04 the method for disabling Ctrl-Alt-Delete restart has change but the documentation at https://help.ubuntu.com/lts/serverguide/console-security.html#disable-ctrl-alt-delete does not reflect this. The files referred to in the documentation don't exist in 16.04 and creating them doesn't help.

[Doug Smythies (dsmythies)]

Evidently, the way to do this now is:

sudo systemctl mask ctrl-alt-del.target
sudo systemctl daemon-reload

I tested it on my 16.04 server, before and after those commands, and it behaved as expected.