Bug 1861 - Unique patrons logins not (totally) enforced

The Koha 3.0 patron admin page will prevent you from creating a user who
has an identical OPAC login as someone else (though I would change the
error message from "Login/password already exists." to "Login
already exists.", since it's not the password that's the
problem).

However, there's nothing enforced in the MySQL database to prevent
patrons from being imported or manually inserted with duplicate logins.
Would it be better to change the MySQL spec from this:
KEY `userid` (`userid`)
to this:
UNIQUE KEY `userid` (`userid`)

Test plan:
1) Enter mysql console
2) Attempt to create two borrowers with identical user id's
3) Note you are able to do so
4) Delete these bororwers
5) Apply this patch
6) Run updatedatabase.pl
7) Repeat step 2
8) Note you are now unable to do so

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Work as described, no koha-qa errors

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Leaves deletedborrowers as it is, deleting patrons still works ok.
Works according to test plan.
Database update will only be successful, when no duplicate entries
exist in the database.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Bug 1861 [QA Followup] - Fix Check_Userid and unit tests

Check_Userid assumes that a borrowernumber will always be passed in
and thus fails to to return 0 for an already used userid when creating
a new patron.

Unit tests must now also me modified to no longer assume it is possible
to create multiple patrons with the same userid.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Bug 1861 [QA Followup] - Don't modify userids, warn if we can't modify the index automatically

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>