Dnctl improvements

Hello,

I opened a review to improve dnctl: https://reviews.freebsd.org/D37431
I don't know how should review it or if I need to add them or not.

There are basically two changes, one to better figure out if executed
binary is ipfw or dnctl, and the other one is to add dnctl rc.d service
so that pf users can easily configure dummynet. I wanted no change for
ipfw users, hence the new rc.d file.

Regards,
meka