hammer2 - Start working on the freemap (note: freemap not yet operational)

* hammer2 show now tries to dump all four volume headers

* Minimum storage allocation chunk size is 1K, increase MIN_RADIX and
  MIN_ALLOC appropriately.

* Preliminary reserved block mappings for the freemap indirect and bitmap
  blocks.

* Preliminary freemap design will use normal indirect blocks (but with
  a different bref.type name).  The check area of the bref will be used to
  store additional freemap hints.

  The volume header will contain a blockref to the root of the freemap,
  and newfs_hammer2 now initializes this blockref.

* Bring major hammer2_disk.h media structure comments up to snuff.

* Formalize blockref.methods and set proper defaults.  This field contains
  the crc/check and compression methods for a blockref.