Input: add userio module

Debugging input devices, specifically laptop touchpads, can be tricky
without having the physical device handy. Here we try to remedy that
with userio. This module allows an application to connect to a character
device provided by the kernel, and emulate any serio device. In
combination with userspace programs that can record PS/2 devices and
replay them through the /dev/userio device, this allows developers to
debug driver issues on the PS/2 level with devices simply by requesting
a recording from the user experiencing the issue without having to have
the physical hardware in front of them.

Signed-off-by: Stephen Chandler Paul <cpaul@redhat.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>