support Debian GNU/kFreeBSD

See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=742275

Note: this is my change I tested with the current Debian package containing v1.11.1. It seems you changed a lot since then so a rebase on master is not possible. A short glance showed me you do not have this in $OS yet, even in master. So maybe you can just include it by yourself. However I leave this here for Debian so they could maybe import my patch into the current package. I guess they won't switch to zfSnap 2.x before you release anything. ;-)

If you think this is not the right approach, please let me know.