[meta] Better IPv6 detection
This issue should be used for brainstorming ideas about IPv6 detection.
The main idea is, that there should be one mechanism that can tell if the user has IPv6 and this information should be shared with init scripts (via uci for example) instead of doing separate detection for every service.
Examples of with detection:
- knot-resolver ping test for disabling IPv6: turris-os-packages:net/knot-resolver/files/kresd.init#L270
- hotplug - https://gitlab.nic.cz/turris/turris-os-packages/-/merge_requests/381/diffs#diff-content-160a8a7b73e11a0e3502870d06ac486732a69d98