style: address a few style issues

- Fix cpplint complaints
- Utilize 80 cols in a few comments / function arg lists, to be
  more consistent with the rest of the codebase
- Header guard: SRC_NGX_GZIP_SETTER_H_ -> NGX_GZIP_SETTER_H_
  Per style guide this was correct, but somehow the rest of the
  code base is wrong :-) Jeff has disabled the header guard lint
  checks, so probably this was by intent.
- Most importantly: changed strncpy -> snprintf as recommended
  by cpplint - please take a good look!