When resource limit is hit, cvc4 outputs useless counterexample. Query
cvc4 for the reason of answer unknown and use the answer to decide
whether resource limit was hit. If it was hit, do not display the
counterexample.

* src/driver/call_provers.{ml|mli}
(parse_prover_run): If the prover answers unknown, get the information
about the reason of this answer.

* src/printer/smtv2.ml
(print_prop_decl): Query solver for the reason of answer unknown.

* src/driver/driver.ml
(load_driver): Initialize Unknown with no information about the reason
of answer unknown.

* src/session/session.ml
(load_result): Initialize Unknown with no information about the reason
of answer unknown.

* src/session/session_scheduler.ml
(schedule_proof_attempt)
(edit_proof): Initialize Unknown with no information about the reason
of answer unknown.

* src/why3session/why3session_lib.ml
(filter_spec): Initialize Unknown with no information about the reason
of answer unknown.