Pass full filepaths to DataListener

[kestrel-x86]

When using a DataListener, DataEvents that include a file name will instead now use the path to the file relative to the FTP root directory.

For example:

match evt {
    DataEvent::Put { path, .. } => {
        /*
        before, `path == "kestrel.jpg"`
        after, `path == "/foo/bar/kestrel.jpg"`
        */
    }
    _ => {},
}

This can potentially be a breaking change.

[kestrel-x86]

Addresses #447