allow launchctl to stop mysql quickly

When launchctl tries to stop a process, it sends INT, waits 20
seconds, and then sends KILL. mysqld happily ignores INT, and just
keeps running until it gets a KILL. That means this patch removes 20
seconds from the shutdown/restart of every machine running mysql via
boxen, which seems like a pretty nice improvement.

I wrote up how I discovered this a while ago:
http://andre.arko.net/2011/08/07/fixing-mac-shutdown-delays/