Require clauses in Z-code COND statements to be either FALSEs or non-empty
LISTs. This fixes an issue where omitting the parens around a clause would cause the FORMs inside to be treated as individual clauses (as if they were lists), producing unexpected behavior and confusing warnings.
Note that clauses undergo macro expansion first, so a FORM can still appear where a clause is expected, as long as it's a macro invocation expanding to a FALSE or non-empty LIST.
[ZILF-212]