Bug 20980: Make mancredit.pl use Koha::Account::add_credit

Bug 20980: Make mancredit.pl use Koha::Account::add_credit

This patch makes creating a manual credit from the UI record the account
offset as 'Manual Credit', and properly set account_offsets.credit_id
instead of account_offsets.debit_id.

To test:
- Create a manual credit (of 'Credit' type) for a known patron (acevedo?)
- Run:
  $ sudo koha-mysql kohadev
  > SELECT * FROM account_offsets;
=> FAIL: The account offset for the manual credit has type=Manual Debit,
    credit_id=NULL and debit_id=accountlines_id
- Run the atomic update:
  $ updatedatabase
- Run:
  $ sudo koha-mysql kohadev
  > SELECT * FROM account_offsets;
=> SUCCESS: The account offset has been corrected and now has
type=Manual Credit, credit_id=accountlines_id and debit_id=NULL
- Create a new manual credit (of 'Forgiven' type) for a known patron
- Run:
  $ sudo koha-mysql kohadev
  > SELECT * FROM account_offsets;
=> SUCCESS: The account offset has been stored correctly as a credit!
- Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>