Repair problem noted by Elphick: make_rels_by_joins failed to handle

Repair problem noted by Elphick: make_rels_by_joins failed to handle
cases where joinclauses were present but some joins have to be made
by cartesian-product join anyway.  An example is
SELECT * FROM a,b,c WHERE (a.f1 + b.f2 + c.f3) = 0;
Even though all the rels have joinclauses, we must join two of them
in cartesian style before we can use the join clause...