IPv6 works on clients with default config, not with "default config"

The changed behaviour is due to to recent changes by @systemcrash
The PR added help texts, and also provided some new detailed config fields for odhcpd.

I noticed the added config items to config, but didn't notice (yet) problems with IPv6.

It is related to odhcpd (not odhcp6c).

Reference to

and the discussion thread (last few messages)