Bug 9576: can now control whether max loans limit can be overriden

Bug 9576: can now control whether max loans limit can be overriden

Adds a new system preference AllowTooManyOverride to control whether
a librarian can override the 'Too many checked out' message which is
currently always overridable.

Test Plan:
1) Apply patch
2) Run updatedatabase.pl
3) Attempt to check out 1 more item to a patron than the max issues
4) You should be allowed to override by default ( current behavior )
5) Set AllowTooManyOverride to "Don't allow"
6) Repeat step 3
7) You should be blocked from being able to issue the item

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
The new system preference is activated by default, which mean there
will be no change in behaviour on update.

The system preference is correctly added to the database and .pref
files.

Test plan and QA script passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>