Handle &key DIRECT-DEFAULT-INITARGS in :AROUND method.

This is the beginning of a migration of the handling of initargs in
:AROUND methods rather than :AFTER methods.  The inconvenience with
the solution using :AFTER methods is that we must either use a writer
generic function for slots that need to be modified, or use
SLOT-VALUE.  If we do it in an :AROUND method, we can modify the
arguments before calling CALL-NEXT-METHOD.  The slot is then
initialized normally by SHARED-INITIALIZE.