hammer2 - Support multiple PFS mounts from the same device

hammer2 - Support multiple PFS mounts from the same device

* Implement hammer2_pfsmount_t and move hmp->rchain, hmp->iroot, and
  a few other fields into the new structure.

  struct mount -> struct hammer2_pfsmount (pmp) -> struct hammer2_mount (hmp).

* Implement mount/umount logic to allow the device to be overloaded.
  Multiple PFS labels associated with the same media may now be mounted.

* Generate a unique pfs_fsid for each PFS created, the {pfs_fsid, pfs_id}
  tuple forms a unique key that the userland helper will use to distinguish
  registrations.