r2791: got rid of talloc_unreference() and instead created talloc_unlink(),

r2791: got rid of talloc_unreference() and instead created talloc_unlink(),
which is much clearer and simpler to use. It removes a specific parent
from a pointer, no matter whether that parent is a "reference" or a
direct parent. This gives complete control over the free process.
(This used to be commit 6c563887f1b9b8c842309a523e88b6f2a32db10f)