Support for graceful reload of mule processes.

- On reload master now signals mule with HUP instead of KILL.
- Added optional call to PyOS_AfterFork to enable cpython child
  processes to override and trap HUP signal.
- Copied cursed_at/no_mercy_at pattern used with worker processes.
- Added mule_reload_mercy option.
Merge pull request #602 from paulegan/mule-graceful-reload

Support for graceful reload of mule processes.