Fix FreeBSD service startup

Fixes #199 

The rc script committed in #143 has some problems; by not exporting `pidfile` at the top-level, it causes the FreeBSD service subsystem to dig around for a PID by using `/bin/ps`, which catches the currently-running Chef client run, thus causing the problem described in #199. By simplifying the rc script, this problem goes away.

Also, there is no need to define a special `chef_stop()` routine. We should just be relying on the service subsystem to do the right thing. Plus sending -9 to Chef is an unnecessarily large hammer; the service subsystem will send `TERM`.

/cc @cwebberOps