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.