Fix FIND-CLASS of compiled-but-not-loaded structure classes
... slightly sucky hook variable in %TARGET-DEFSTRUCT, to be
used by PCL
... ENSURE-NON-STANDARD-CLASS updated to cope with the
possibility that a CLASSOID exists but the accessor
functions aren't FBOUNDP.
... (side benefit: redefinitions of structures are now reflected
in PCL classes)
... test for FIND-CLASS non-breakage.