arp_poison_request by default

This pull request is based on our recent discussion, to enable ARP poisoning by request by default.
We've found this method being introduced 10 years ago by alor after a developers meeting but since then remained disabled by default.

Maybe @alor had a reason for keeping it off, which would be good to know then.

This pull request isn't yet finished and ready to merge but can be used as a discussion point.
Testers are well appreciated.