evdev: export event device properties through sysctl interface

evdev: export event device properties through sysctl interface

A big security advantage of Wayland is not allowing applications to read
input devices all the time. Having /dev/input/* accessible to the user
account subverts this advantage.

libudev-devd was opening the evdev devices to detect their types (mouse,
keyboard, touchpad, etc). This don't work if /dev/input/* is inaccessible.
With the kernel exposing this information as sysctls (kern.evdev.input.*),
we can work w/o /dev/input/* access, preserving the Wayland security model.

Submitted by:		Greg V <greg@unrelenting.technology>
Reviewed by:		wulf, imp
MFC after:		2 weeks
Differential Revision:	https://reviews.freebsd.org/D18694