kernel - Allow multiple opens on /dev/udev

kernel - Allow multiple opens on /dev/udev

* Implement as an auto-clone device

* Refactor most of the code to use a new queueing mechanic.  There is a single
  queue and multiple markers pointing into the queue (one for each initiated
  open'd device).

* Events broadcast to all initiated readers.  We use a single-queue /
  multiple-marker model.  The event structure is only deleted once all markers
  have iterated past it.