[no sq.] Make client use second/fallback IP address from DNS

[sfan5]

For a problem description see this Wikipedia article but note I am not implementing what is written there.
Minetest will simply try the second address if it hasn't received any reply after 1.8s. This is invisible to the user.

tl;dr You can now host a dual-stack server without risking that a client will prefer IPv6, fail to connect and never try via IPv4 which would have worked fine.

To do

This PR is Ready for Review.

How to test

For one, go test the normal stuff like singleplayer or connecting to a remote server.

To test fallback:

1. ensure you have ipv6 and your host prefers it
2. Run ./bin/minetest --go --name test --address test14200.nekomimi.pw --verbose
3. Observe nothing happening for ~2s, this is when it tries IPv6
4. After that time you will receive an error because the IPv4 points to some random 0.4.x server I pulled from the list

[SmallJoker]

Attempted to join a few old servers, followed by compatible ones. Works as expected. However, I did not notice that any fallback was needed during these tests. The code however looks good.