Reuse unlink inside of rename

[jkominek]

If you rename one file on top of another existing file, you've got to do much of the work of the unlink operation before you can finish the rename. That takes place in Inflight_rename::complicated, but we might be able to invoke unlink, somehow, and make use of it. There'd probably need to be another entrypoint to unlink which let us pass in/use an existing transaction, as that needs to take place under the rename's transaction.