hammer2 - Refactor file unlink w/open descriptor (3)

* Fix a bug where hammer2_inode_connect() was not calling
  hammer2_chain_lookup() to properly adjust the parent chain pointer
  to the correct insertion point when moving an unlinked-but-open file
  to the hidden directory.

  (triggered with poudriere which uses open/unlinked filesystem FIFO nodes).

* Minor documentation cleanup.