hammer2 - dmsg blockdev work

hammer2 - dmsg blockdev work

* Adjust the LNK_CONN and LNK_SPAN messages to provide a cluster label
  and a filesystem label instead of just a filesystem label.

* Change the cluster controller to index clusters based on the
  peer_type, pfs_clid, and cl_label instead of just the pfs_clid.

  HAMMER2 PFS's are primarily identified by pfs_clid and unique nodes
  are identified by pfs_fsid.  fs_label holds the HAMMER2 super-root
  name.

  BLOCK devices are primarily identified by cl_label and unique nodes
  are identified by pfs_fsid.  pfs_clid and fs_label are empty.

* Adjust the cluster controller's matching filters such that, e.g.
  BLOCK device clients only have visibility to the BLOCK device server
  for the cl_label in question, and do not have visibility to other
  BLOCK device clients.

* Flesh out the hammer2 service daemons disk monitoring, have it attach
  a pipe to the in-kernel disk via an ioctl().

* Continued work on the skeleton network disk exporter (kern/subr_disk*) and
  network disk importer (dev/disk/xdisk/*).  xdisk has not been added to the
  build yet.

* Move a few more definitions from HAMMER2 to DMSG.