Check that everyone doesn't have the same lastip address

Found as part of MDL-69074

Default installed now have a more secure setting, in theory, but a narrow value for getremoteaddrconf is just as insecure as a wide value because everyone will end up with the same ip.

So proposing to improve the getremoteaddrconf by looking at the last N users who logged and seeing if they share the same ip address, and if there is a very high degree of overlap that is a heuristic that the setting is wrong.

There are some use cases like labs and exams where lots of people will legitimately have the same ip so this can never be perfect.

  SELECT count(*), lastip

    FROM (

             SELECT lastip

               FROM {user}

           ORDER BY lastlogin DESC limit 1000

         ) AS ips

GROUP BY lastip

ORDER BY count(*) DESC, lastip