Investigate whether IPv6 link-local addresses work over consumer wifi networks

Meshenger uses IPv6 link-local addresses to communicate with peers on the same LAN. A device's link-local address is derived from its MAC address, so it's the same on every LAN.

If this works reliably on consumer wifi networks it would have a major advantage over our current approach: contacts that connect to a new network would be able to connect to each other immediately without first exchanging updated transport properties via some other transport.

If it works when one of the devices is providing a wifi hotspot, it might also provide an alternative to #1328 (closed).

Related to #28, #1193 (closed), #1328 (closed).