Bug 12197: enforce the maxreserves preference when staff members place hold requests

Bug 12197: enforce the maxreserves preference when staff members place hold requests

This patch ensures that the global maxreserves preference is enforced
when staff members place hold requests.

For example:

Create 3 items to place holds on. Set the circulation rule to allow 50
holds for all items. Set maxreserves to 2. Place a hold on 3 different
items. On the third item, it will give a warning, but you can still
place the hold. Despite what the circulation rule is set for (which is
only a specific case rule), maxreserves is a global rule and
should stop this from happening, not just give a warning.

Test Plan:
1) Reproduce the bug by following the steps above
2) Verify the bug exists
3) Apply this patch
4) Verify the librarian cannot place the hold now
5) Enable AllowHoldPolicyOverride
6) Verify the librarian can forcefully place the hold

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 12197: (follow-up) rename variable for greater clarity

"maxreserves" was referring both to the system preference and to the
condition of having exceeded the number of hold requests allowed.

This patch renames a variable to remove the ambguity.

Test plan:

* Same as the main patch.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>