RDNSS (RFC5006) support with rtadvd/rtsold

[Bernd Walter]

It seems our rtadvd is missing support for stateles DNS configuration.
There is also a NetBSD PR about this (41510), so there is likely
no code to get from them.
How difficult would it be to add RFC5006 support to at least rtadvd?

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.