hammer2 - Starting refactoring PFS management in mount

hammer2 - Starting refactoring PFS management in mount

* Start removing single-device shims.

* Adjust data structures.  Rename hammer2_mount to hammer2_dev and
  rename hammer2_pfsmount to hammer2_pfs.  Refactor unmount.

* Integrate all available PFSs when a block device is mounted and
  De-integrate related PFSs when a block device is unmounted.

  At least one PFS (typically @LOCAL) must be mounted from a HAMMER2 block
  device for that device's PFSs to be used.  At least for now.  We could
  eventually trigger auto-mounting via the probe code.

* Add a synchronization thread abstraction for a PFS.

* Normalize chain->pmp to NULL for any chain which is part of the super-root
  topology, for later sanity assertions.

* The ioctl to create a PFS now adds it (delete and snapshot do not, yet).