Remove ifb kernel module on teardown

[goodboy]

I've noticed that netimpair neglects to delete the ifb0 and ifb1 interfaces added by modprobe ifb (i.e. never unloads the kernel module).

I think it would be sensible to detect if the module is loaded at start (modprobe --first-time ifb will error if it is) and then run modprobe -r ifb on tear down if the module wasn't active at start.

This preserves the system state on target machines after netimpair has done it's dirty work ;)