Update sets dynamically

[qdeslandes]

Currently, users define sets in their ruleset. Once the ruleset (or chain) is applied, the set's content can't be modified unless the ruleset (or chain) is re-generated. bpfilter should allow users to update only the set's content, while keeping the same rules definitions.
Maps used by bpfilter are sized during generation, meaning the whole map will have to be recreated. Luckily, bpfilter contains all the data required to transparently re-generate and update a chain.

bpfilter should provide a new API to update a set. Under the hood, this call would reuse an existing chain, copy it, update its map, regenerate it, and update the attached BPF objects.

[pzmarzly]

Done in #404