Send SIGQUIT once when stopping ungracefully.

Sending SIGQUIT multiple times to child processes can trigger issue #14173
when running in python < 2.7.5.

Fixes #655
Merge branch 'master' of https://github.com/gcallaghan/gunicorn

* 'master' of https://github.com/gcallaghan/gunicorn:
  Send SIGQUIT once when stopping ungracefully.