Cosmetic

Remove outdated comments, add some new ones, re-indent.
Add nslhelp.api to default tests

If we keep nsl (which takes 10-15mn) we should have nslhelp (about 30s) too.
Fix bug in runtests

Generated files should not be copied and marked as unwritable, otherwise they
cannot be regenerated by make inside the test subdirectory.
Clarify and slightly enhance clause freshening

Freshen before running conseq, to avoid having the same variables in the
clause being checked for conseq and in clauses from the kb. Maude seems to
handle that case nicely, but better be safe.

Additionnally, fine tune where freshening occurs: by doing it before
normalization we benefit more from the "canonizing" effect of the latter
transformation, which identifies more clauses and leads to slightly smaller
knowledge bases.

This modification does not seem to have a performance impact on make test.
In some cases it generates one less tester.
More robust ordering of clauses in Equation

Do not rely on the id of clauses, but check if one is the plus clause.
Tests still run and it does not seem to make a difference.

Also remove unused parameter ?rx'. There is no point to it anyway since
we now forbid equation between two clauses with plus recipes.