Port to Conduit 0.6.0 API

```
CONDUIT_DEBUG=1 ./_build/bin/ogit.native clone git://github.com/mirage/mirage --verbose
```

gets as far as 

```
2014-11-20 14:24:49.135 remote       DEBUG: EXPANDING THE FILESYSTEM
Unix.Unix_error(Unix.EBADF, "check_descriptor", "")
```