hammer2 - Revamp hammer2_cluster structure part 1

hammer2 - Revamp hammer2_cluster structure part 1

* Adjust the hammer2_cluster structure to track multiple chains (and thus
  multiple mount points).  Use cases still just pull out one chain.

* Remove schain from hammer2_mount, leaving just sroot.  The cached chain
  structure isn't useful because it doesn't track delete-duplicate
  operations.  Use sroot for basing functions.

* Mark pfs-mount chains with the PFSROOT flag and propagate through
  duplications to detect when a snapshot should inherit the PFS
  cluster id or not.