hammer2 - Adjust chain lookup/iteration API, add readdir

hammer2 - Adjust chain lookup/iteration API, add readdir

* Adjust the chain lookup/iteration API to take a key range key_beg to key_end
  (inclusive), instead of a key/mask pair.

* Add and initialize a parent inode pointer in hammer2_inode{}.

* Adjust the directory key hash to always OR with 0x8000U in order to
  guarantee that some low valued codes remain unused so we can use them
  for pseudo directory cookies (for '.' and '..').

  newfs2_hammer is required on the test volume with this change.

* Implement hammer2_vop_readdir().