Establishing standard return values for questions.

Providing a "good" documentation base for new questions, and tuning the
pre-existent ones to the adapted model.
Specific changes:
* return defined (and fixed) in setup, teardown, test, ask_rsa
* documenting example question, questions.h's qa_question_t
* prettify interface, displaying error and success messages in stderr.
* qualifying the RSA* argument in qa_question_t.ask_rsa to const.