hammer2 - More work on userland hammer2 msg infrastructure

hammer2 - More work on userland hammer2 msg infrastructure

* When a link error occurs generate a LNK_ERROR message for each transaction
  before setting the iocom ERROR flag and returning the final
  non-transactional LNK_ERROR.

* Processing command switches switch on the original transactional head.cmd
  instead of the current msg->any.head.cmd, which allows the use of mixed
  cmd's in a transactional message stream.

  The target function then handles the actual msg->any.head.cmd.  Thus we
  can consolidate all sub-commands used within a transaction into the target
  function, which greatly improves code quality.

  This allows us to send LNK_ERROR messages over active transactions.

* Print the pfs_id and label for the received LNK_SPAN message the kernel
  sends to the userland hammer2 service process, and verify LNK_ERROR
  processing for connection terminations.  Yup, it works.