[p6object] If a class already does P6object, don't try to add it again. This enables us to register derived metaclasses.
[p6object] If a class already does P6object, don't try to add it again. This enables us to register derived metaclasses.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@40255 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot