hammer2 - GSOC cleanup pass

hammer2 - GSOC cleanup pass

* Cleanup some syntax and convert // comments in core source files.
  Remove _t suffix on static compression type procedures, make a
  few variable names shorter, etc.

* Used in-memory hammer2_inode->comp_heuristic instead of the on-media
  ipdata->reserved85 field for the compression heuristic.  It doesn't
  need to be recorded on-media.

* If the write thread encounters an I/O error, flag the original logical
  bp with the error and continue rather than breaking out and killing
  the write thread.