ssh/terminal: support ^N and ^P

This makes it possible to navigate the history without leaving
the home row on the keyboard.

Change-Id: Id24c43f8eb6090520ab37bf8126264901b70c489
Reviewed-on: https://go-review.googlesource.com/c/33618
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>