added killsnoop, another snooping tool for tracing kill()s via ftrace

Hi Brendan,

first at all, your tools are awesome and excellent for learning kernel internals and for debugging problems.
I've added killsnoop for snopping kills, since I recently had the problem, that someone kills a worker application, which we where not able to locate the origin. In the end, it was a cleaning cron job, which kills long running (and hanging) processes :-)

Maybe someday, it will be useful for someone else.

Best regards,
Martin