Make dummynet kernel module independent from IPFW

[meka_tilda.center]

Summary

The dummynet module has no reason to depend on IPFW, while the rest of the code can benefit from dummynet, like adding support to PF.

[thj]

Is this still relevant to your porting work? If I get an ack I will add this to my list to look at.

[meka_tilda.center]

Yes, it's still relevant. I'm very slow at progress on PF+Dummynet integration, but I am working on it, and dummynet not depending on IPFW is essential.