Bug 8251 Do not try to debar patrons if returns are not overdue

Bug 8251 Do not try to debar patrons if returns are not overdue

If a period of suspension is configured in the issuing rules
a calculation to debar the patron was called on all returns
It should be limited to overdue returns

Renamed _FixFineDaysOnReturn subroutine to _debar_user_on_return
which is more descriptive of its purpose
Removed some unnecessary or duplicated processing
Changed visibility of $today so it didnt need
calculating twice
Removed declaration of a datedue variable that is
never used

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>