Chef 4357

- Don't display stacktraces when client fork is enabled unless log level is debug
- Mutate argv0 in the worker process to show helpful info
- improve error message when reaping failed worker process
- Override Chef's default signal handlers for int/term in worker (worker exits immediately)
- There may be a small number of failure cases where we would catch and run `debug_stacktrace` where we don't with this patch (anything in Chef::Client#initialize), but rerunning chef with `-ldebug` ought to show the stacktrace, so I think the downside should be minimal


http://tickets.opscode.com/browse/CHEF-4357