rewrite add command to use dagwriter

moved a pinner into the dagwriter for inline pinning
Also, seems like you could just pass the DagWriter in directly. As in, maybe this function should just be taking a dagwriter.

```Go

dw, err := uio.NewDagWriter(dserv, chunk.DefaultSplitter)
if err != nil { ... }
nd, err := import.ImportFileDag(filename, dw) // arg lint error.
```