Multipart File Transfer

This PR is ready for CR.

Overview of changes:

* Added a `commands.File` interface that implements `io.Reader` (which also has a filename and can be read in parts)
* Changed from accessing file argument values in `Request#Arguments()` to a separate `Request#Files()`
* Open files recursively on the client (and make them into `commands.File`s)
* Transfer files as multipart in `commands/http`
That's not a bug since `io.EOF` only gets returned when `written == 0` (but my code did make that confusing).

I cleaned up the flow based on your suggestions: 6630103