Call payment processor's checkConfig method on initial configuration

CRM_Admin_Form_PaymentProcessor::buildQuickForm() validates that "user_name" is present, but this is not valid for all payment processors (eg those which configure via URL instead of username). There is a comment on the form rule which says "would be good to make this payment processor specific".

CiviCRM payment processors are permitted to implement their own validation via the checkConfig() method, so the suggestion in that comment appears to have been implemented.

In the case of the F2C Donate payment processor, the presence of this form rule requires that a redundant field be retained and populated by the user.