hammer2 - Revise newfs_hammer2

hammer2 - Revise newfs_hammer2

* Bring newfs_hammer2 up-to-date with the cluster mechanics I intend to use
  for the filesytem.

  Generally change the default label from "ROOT" to "LOCAL", the intention
  being that all formatted volumes will at least mount device@LOCAL in order
  to make a HAMMER2 volume available to the cluster protocol.  Once the
  volume is made available, the intension is to then allow other PFSs can
  be created/deleted/managed via the cluster protocol.

  newfs_hammer2 will also create a BOOT and ROOT PFS by default, intended for
  simple non-clustered configurations, and the -L label option works
  differently now.

* Generally speaking, all block devices to be used in the cluster must be
  formatted with newfs_hammer2.  A formatted HAMMER2 volume can be used to
  host any number of PFSs (Pseudo-FileSystems).  Each PFS is independent
  and can be part of different clusters.