nsf.c:
- cleanup of NsfParameterInvalidateClassCacheCmd(): performance improvements.
  After the fixing of indirect mixin validation, performance of invalidation
  went up by a factor of 5. At least, in some important cases (invalidation of
  rootclasses like nx::Object / xotcl::Object), we are again on the same level
  as before (actually slightly faster).
- use functions IsRootClass(), IsRootMetaClass() and IsBaseClass()
  to access object/class properties
- add gcc attribute "pure"