hammer2 - wire in statfs/statvfs, add the chain structure, etc

hammer2 - wire in statfs/statvfs, add the chain structure, etc

* Wire mnt_stat and mnt_vstat.  Currently dummy-up the values.  This allows
  the mount point to show up in 'df' output.

* Add the chain structure for upcoming directory search work.  This
  structure will also be used to track through indirect blocks.  The
  chain structure supplies the full blockref chain from an in-memory
  structure to the root volume, allowing modifications to be properly
  tracked.

* Move the volume header read to its own support procedure, it's going to
  eventually be considerably more complex.

* Bring the hammer2_dirhash() procedure in from newfs_hammer2.