Separate parallel_classify_warnings out of parse_input_file.

* Allow other callers of this module to:
  * pass any input stream to parse_input_file,
  * pass any warning_lines to parallel_classify_warnings,
  * call dump_csv or dump_html to get output.
* No output change.
* Capture and ignore signal.SIGTERM at the end,
  to avoid bad warning/error messages from the exit clean-up process.

Test: run warn.py build.log.
Change-Id: I1414797a536c0ee622e2a34c226578621be1ddab