nsf.c:
- new function DependentSubClasses() to determine all classes that inherit
  from a given class. The result extend TransitiveSubClasses() by including
  class mixin relationships.
- simplify NsfParameterCacheClassInvalidateCmd() by using the new function
  DependentSubClasses(). With the new implementation,
  NsfParameterCacheClassInvalidateCmd() is as efficient as before without
  to MostGeneralSuperclass optimization (but being complete now) when
  working on the root classes (an more efficient on subclasses).