ScpClient: Uploading and Downloading events should provide absolute local and remote path

Problem

Right now, the ScpDownloadEventArgs and ScpUploadEventArgs classes only have a single Filename property that contains the only file name of the local or remote file that this being downloaded or download.

For the Download overloads, ScpDownloadEventArgs.Filename always contains the file name of the remote file that is being downloaded.

For the Upload overloads, there's little consistency in the value of ScpUploadEventArgs.Filename:

• For Upload(FileInfo fileInfo, string path), its value is the file name of the local file.
• For Upload(Stream source, string path) and Upload(DirectoryInfo directoryInfo, string path), its value is the file name of the remote file (which is the only path available).