cluster - Major kernel component work (diskiocom, xdisk, kdmsg)

cluster - Major kernel component work (diskiocom, xdisk, kdmsg)

* Major implementation and stabilization work.  Fix lots of bugs,
  improve the AUTO flags and APIs.

* xdisk and diskiocom are now operational for the first time.

  xdisk is a consumer of remote block devices.  You pass it one end of
  a pipe and use an ioctl to create /dev/xa* devices.  The kernel's
  nominal disk management subsystem handles /dev/serno/* and will also
  probe the /dev/xa* devices to create the slices.

  diskiocom is a block device service.  An ioctl is used to connect a
  raw disk device to the network via a pipe.

* The hammer2 service demon ties the two together over the network.

* Initial testing with a local hammer2 service daemon looping da0 to xa0
  succeeded, ~46MB/sec unoptimized using dd.