kernel - Preliminary xdisk remote block driver for cluster

kernel - Preliminary xdisk remote block driver for cluster

* Basic infrastructure, ties into kern_dmsg.c cluster messaging system.
  (still a skeleton).

* Allows a userland program (aka hammer2 service) to add networked block
  devices to the local machine.

• [DBH] sys/dev/disk/xdisk/xdisk.c
• [DBH] sys/sys/xdiskioctl.h