Enhancement: parse special offsets in filter expression

[fenner]

#861 talks about output, but input is also important. I would much rather say "to look at skb_type you use ether[type]=3" instead of "ether[-4092]=3" and have to figure out if that is the value I really meant.

I'd be fine (it might be better, even) if it was introduced by a different value - special[type]? skf_ad[type]? (I'm bad at naming!)

[guyharris]

I might vote for skf_ad to remind users that it's for Linux SKF_AD_ values.

The compile would fail unless you're compiling for a live capture on a PF_PACKET socket.