autodetection : simplify the semantics of the file provers-detection-data.conf

* all the provers with the same prover_id are threated as one prover,
* the first description of a prover in the file pdd.conf which have an
  executable with a good version is used,
* the order used to test the executable (exec) inside a prover description
  is not specified,
* a version_bad allows to forbid a version.
* a version_ok matches no warning is printed
* a version_old matches a warning is printed
* if no version matches a warning is printed