hammer2 - Refactor frontend part 16/many

hammer2 - Refactor frontend part 16/many

* Replace cluster ops with more frontend/backend XOPs.  Use XOPs to
  resolve the root inode, handle the hidden directory, create the
  hidden directory, and to run the unlinkq in the hidden directory.

* Cleanup additional cluster ops that can use ip->meta instead.

* Simplify hammer2_inode_unlock(), it is no longer cluster-centric so
  do not pass-in a cluster to unlock and drop.

* hammer2 show - dump pfs fields for the super-root

* hammer2 set* directives, change the HAMMER2IOC_INODE_SET API slightly.

* newfs_hammer2 - name the super root directory for show output convenience.