Implemented support for ILL requests in VuFind core code and VoyagerRest...

...ful and Demo drivers.

Identical language strings:
ill_request_available = hold_available
ill_request_cancel_fail = hold_cancel_fail
ill_request_cancel_success = hold_cancel_success
ill_request_cancel_success_items = hold_cancel_success_items
ill_request_date_invalid = hold_date_invalid
ill_request_date_past = hold_date_past
ill_request_error_fail = hold_error_fail
ill_request_invalid_pickup = hold_invalid_pickup
ill_request_place_fail_missing = hold_place_fail_missing
ill_request_place_success = hold_place_success