Introduced alternative reader INITARGS like SLOT-DEFINITION-INITARGS.

Unfortunately, the accessors for direct and effective slot definitions
are the same.  This is unfortunate because of problems during
bootstrapping.  Direct slot definitions are accessed during class
finalization in order to create effective slot definitions and
effective slot definitions are needed for initializing instances, so
they are needed at the same time.  In Phase 4 of bootstrapping, the
direct slot definitions that are used in class finalization are target
instances, but the effective slot definitions needed for initializing
instances are bridge instances.

The solution (which is not pretty) is to define a set of alternative
readers for slot-definition objects and make sure that during phase 4,
the alternative readers are used in class finalization.