HAMMER UTIL - Version 4 part 2/many - UNDO FIFO layout work.

HAMMER UTIL - Version 4 part 2/many - UNDO FIFO layout work.

* newfs_hammer now formats the UNDO FIFO space to conform with version 4
  filesystems.

* newfs_hammer now creates a version 3 filesystem by default (instead of 2).

* Fix bug in blockmap lookup code.  The wrong hammer offset was being used
  to validate the freemap zone.  It only effected UNDO zone offsets.

* Add the 'show-undo' directive which dumps the UNDO FIFO space.