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`
This is just the exit point for when the data has all been read (to make callers will stop calling `Read`).

The reason it needs to check if `mfr.buf.Len() == 0` is so we can ensure the final data in the buffer has been flushed.