[atlas] Probe DHCPv6 support

In your letter dated Tue, 9 May 2023 19:17:41 +0200 you wrote:
>That you do not see more of it is mostly Android's fault - because
>enterprises get told "it will not work with half the mobile devices",
>so they go and postpone IPv6 deployment instead (in places where the
>control bit is strictly required)...

Android devices show up at relatively untrusted wifi networks. I can see
the problem there. Or are you saying the these days enterprises are
100% cloud based, and the only networks they have left are wifi?

In many cases untrusted devices only communicate with the internal network over a VPN. In that case, the local access protocol doesn't matter much. The VPN
can hand out IPv6 address even if the host is IPv4-only.

In any case, Atlas probes run a stripped down version of OpenWRT. It may help
if somebody can figure out which OpenWRT packages need to be added to 
enable DHCPv6.