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`
@mappum I keep finding trouble because the cmds lib only handles paths relative to the root, i.e. treats the root as an inaccessible command. I think changing this (so that path is the full path, including root, and root is addressable, will make lots of special casing go away.