Bug 10577: Improve semantics of GetBudgetPeriod()

Bug 10577: Improve semantics of GetBudgetPeriod()

Remove the option to pass zero to this function in
order to get "the" active budget.  This was a problem
in three ways:

- Koha doesn't require that there be only one active
  budget at a time, so the concept of "the" active
  budget doesn't make sense.
- Having the single parameter be either an ID or a flag
  based on its value is poor function design.
- No callers of GetBudgetPeriod() were actually using this
  modality.

This patch also improves the DB-dependent tests for budgets by

- wrapping the test in a transaction
- counting budgets correctly

To test:

[1] Apply the patch.
[2] Verify that prove -v t/db_dependent/Budgets.t passes
[3] Verify in the staff interface that:
    - the budget hierarchy displays correctly
    - you can add and modify a budget

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Rescued-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>