hammer2 - Implement most of the hammer2_chain_*() function set

hammer2 - Implement most of the hammer2_chain_*() function set

* Implement hammer2_chain_push/first/next/get/link/unlink.  Integrate
  the hammer2_inode into the chain topology.

* Route both the reference count and the busy count through the chain
  structure.

* Implement simplified I/O via the chain functions.

* Indirect blocks not yet implemented in the chain functions.

* Use the chain functions to locate both the super-root inode and the
  root-inode via its label.