Bug 6906: Follow coding guidelines in terminology.

Bug 6906: Follow coding guidelines in terminology.

* Koha/Patron.pm (wantsCheckPrevCheckout, doCheckPrevCheckout): Rename
  to...
  (wants_check_for_previous_checkout, do_check_for_previous_checkout):
  ...this.
* C4/Circulation.pm: Use new names.
* t/db_dependent/Patron/CheckPrevCheckout.t: Renamed to...
* t/db_dependent/Patron/Borrower_PrevCheckout.t: ...this.

Bug 6906: Fix POD reference to old method name.

* Koha/Patron.pm (wants_check_for_previous_checkout): Fix POD.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>