#340 multiple users cause failure in upgrade

[Jams H Thompson]

TikiWiki DB can contain multiple user accounts with the
same ID. This causes the upgrade to fail in various
ways.

Upgrade should execute the following query: select
login from users_users uu group by (uu.login) HAVING
count(uu.login) > 1 order by binary login desc;
and if any rows are returned, then fail with an error
message directing user to fix problem before proceeding.