hammer2 - cluster API, preliminary implementation of multi-chain updates

* Separate out read-only from read-write access to cluster data.

* Implement hammer2_cluster_modsync() and call in appropriate places.

* Clean up some media inode fields not currently in-use.
hammer2 - skeleton multi target mount detection

* Detect H2 filesystems with the same cluster id and add to an existing
  mount's cluster instead of mounting separately.

* Very preliminary, not operational.  Transactions, flush, and volume
  management need to be rewritten.
newfs_hammer2 - add -R uuid and -I uuid

* Allow the super-root (-R) cluster id and the labeled PFS's cluster id
  to be specified.

* Populate the cluster id in the inode data.
mount_hammer2 - Check ERANGE for mount integration result

* For now  the H2 vfs code returns ERANGE when mounting multiple
  elements of a cluster, report it as an integration instead of
  as an error.