kernel - Bring in evdev from FreeBSD

kernel - Bring in evdev from FreeBSD

* This is a port of evdev, the generic input event interface, from
  FreeBSD. Wikipedia: "Evdev generalizes raw input events from device
  drivers and makes them available through character devices in the
  /dev/input/ directory."

* It is currently considered experimental since in rare cases it may cause
  kernel crash when a device, e.g. usb mouse or keyboard, is detached
  while a userland program reading from the corresponding input/eventX
  device.

* In order to enable evdev, kernel needs to be rebuilt with 'device evdev'
  and 'options EVDEV_SUPPORT'. For debugging, add 'options EVDEV_DEBUG'.

* At present, only ums, kbd and kbdmux can send events to their respective
  input/eventX devices. More drivers will be added in due course. The sysctl
  kern.evdev.rcpt_mask determines which drivers send events to evdev.