Add read-only SLAPI_X_ADD_STRUCTURAL_CLASS pblock extension.

Add read-only SLAPI_X_ADD_STRUCTURAL_CLASS pblock extension.

This is necessary to compensate for a regression in the SLAPI
implementation since it moved to an overlay: global add preop
plugins no longer have access to operational attributes,
because slap_mods_opattrs() is called by fe_op_add(), invoked
after the global SLAPI overlay.

(Some of our plugins need to the structural object class of the
to-be-added entry.)

I suppose an option would be to have the SLAPI overlay
optimistically call slap_mods_opattrs() (as long as this can be
called idempotently). If there are any other ideas let me know.