LP 1198465: Refactor logic into gatekeeper functions

LP 1198465: Refactor logic into gatekeeper functions

The bulk of this commits take the logic from adjust_bills_to_zero() and
moves it up a layer into the "gatekeeper" void_or_zero* functions.
This move also allows us to simplify the logic, since some facts are
already known based on our function path.

Also:
- give void_or_zero_overdues() a new signature to better support
  multiple options
- add new 'force_void' and 'force_zero' options to this function
- rename real_void_bills() to simply void_bills() (since there is no
  other void_bills(), the "real" was redundant)

Signed-off-by: Dan Wells <dbw2@calvin.edu>