Prevent time limits to grow out of control and do not update them behind the user's back.
Prevent time limits to grow out of control and do not update them behind the user's back.

With the previous code, one could start with an original time limit of a
few seconds and end up with proof attempts running for several minutes
after a few unsuccessful tries with provers that are aware of the actual
limit. Now, the actual time limit is never more than twice the user-defined
one.

ssaavedra/why3,jpdeplaix/why3,jpdeplaix/why3,douglas-larocca/why3,ssaavedra/why3,douglas-larocca/why3,douglas-larocca/why3,ssaavedra/why3,jpdeplaix/why3