hammer2 - Make the CRC check code programmable

* Add infrastructure to allow a CRC check code method request to be stored
  in an inode.  The method will be inherited by anything created under the
  inode.

* Refactor the check_algo and comp_algo encoding to make the distinction
  between the requests in ipdata fields and the actual specification stored
  in bref.methods.

* Make sure that the hidden directory inherits the iroot's algorithm
  specifications for consistency.
hammer2 - Add directives for setting the check mode

* Add setcheck, setcrc32, and a few other directives for setting the
  crc check mode on a directory or file.  The inode and any new chains
  or modifications will inherit the mode.

* Make adjustments to stat and show to display more information.
newfs_hammer2 - Set default compression and check modes

* When creating a new filesystem, make sure the directory inodes contain
  proper defaults for the compression and crc check mode.