Change of strategy: don't pass realpath-ed path to client side.
We still use realpath to validate the path up front, but we pass the original (DB) path on to the client-side. Passing the resolved path was wrong anyway for clients that write images across NFS, because the path the client uses could be different than that computed on the server (e.g., /proj/foo vs. /.amd_mnt/ops/proj/foo) due to the way mounts are done. Note that the server will again validate the client-provided path, so if someone were to mess with a symlink in the path between when create_image verifies it and when it gets used, there is still no danger. This will probably eliminate the need for the AMD hack, but I'll leave it just to be safe.