Adds support for input event interface in kbdmux(4) driver
Now it`s possible to replace xf86-input-keyboard driver with xf86-input-evdev for xorg-server keyboard input.
Xorg-server should be patched in that case to prevent interference with console output.
Patch is extracted from xf86-input-keyboard driver and can be found here: https://reviews.freebsd.org/differential/changeset/?ref=186671
Known bug: Keymap reading/writing is not implemented