Bug 13909: Suspension days calculation should respect finesCalendar

Bug 13909: Suspension days calculation should respect finesCalendar

This patch make _debar_user_on_return respect the finesCalendar syspref.

It does so, by replacing the ad-hoc overdue days calculation in favor of
C4::Overdues::_get_chargeable_units (which is renamed C4::Overdues::get_chargeable_units
and exported). There's no behaviour change besides making the calculation simpler
and correct.

To test:
- Set finesCalendar = "directly"
- Have a circulation rule stating:
  interval for calculating fines = 1
  suspension days = 3
- Have the calendar set for sunday and saturday as holidays.
- Checkout an item with a branch/itype/borrower category that matches the defined circ rule with a hand-writen due date to (say) last friday.
- Check the item in
=> FAIL: Notice that the user is debarred using the calendar (skipping saturday and sunday).
- Apply the patch
- Repeat the previous steps
=> SUCCESS: calculation is correct (counting saturday and sunday as overdue days, i.e. 'directly').
- Set finesCalendar = "calendar"
- Repeat the test
=> SUCCESS: calculation is correct (skipping holidays).
- Sign off.

Sponsored-by: Universidad Nacional de Cordoba
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>