Checking that n from parameter-line is at most MAX_VARS.
Correction: auxiliary array "order" must contain only relevant variables.
Checking whether literals have a correct value.
Code simplification by using std::vector.

Removed one leak.
Now checking whether the real number of clauses is at most what the header says.

And cleaning up counting of clauses.
Returning with error in case of empty clause.

Previous behaviour of just stopping reading can be very misleading.
Using std::vector for unit-clauses stack.
Using std::vector for "clauses".
Using std::vector (together with std::array) for "vars".
Using std::vector for n_changes, checker, out, and removing MAX_VARS.

Now arbitrary clause-sets can be handled, as long as the clauses aren't
too long.