Fixed: too few split names were considered (again!).

The fix from f46b9745 was insufficient.

One plus name for all splits is not sufficient.
For two constants a, b, (a = b) is not valid.
To see that on split level 2, at least two names must be considered.

So the new fix is a combination of the pre-f46b9745 state and the f46b9745-fix.

Another, simpler way to deal with split names is probably to assume
that the query only mentions quasi-primitive literals. Any
non-quasi-primitive clause could be brought into this normal form by
replacing (f(t) = g(t)) by (g(t) != x v f(t) = x) and
(f(t) = x) by (t != y v f(y) = x).
The solution is simple because it makes
 - Grounder::Flatten (although we should recycle it in Formula::NF()),
 - Grounder::PlusSplitNames(),
 - Solver::ResolveDeterminedTerms()
obsolete. As we then drop Solver::ResolveDeterminedTerms(), the
reasoner would even preserve soundness (if we keep it, the reasoner
would still fall into the (c = d) trap for k >= 2).