pkttables finally making some progress

I've found some time to work on pkttables again. Isn't that great news? If my brain is not completely broken, I've now worked out a RCU-powered way to have full table traversal with a completely lock-less reader path, while providing atomicity either on table- or chain level.

Also, I ripped the "struct nf_attr" and NFA_xx macros from the nfnetlink core, since they get replaced by my vTLV (Versioned TLV) code.

With some luck I'll be able to continue my pkttables work next week