Implement IPv6 RA parameters

[ipspace]

So far, netlab had no knobs to configure IPv6 RA parameters on the edge subnets. With the introduction of #1710 we will need a mechanism to figure out what to do with RA instead of hoping for the best ;)

Devices not implementing the new framework will be assumed to send RAs with "use SLAAC" bits.

Devices supporting RA parameters should implement these guidelines:

• ra.disable disables RA when set to true
• ra.slaac controls SLAAC
• ra.dhcp controls DHCPv6 usage (IA_NA address allocation or just other configuration)
• ra.onlink controls whether the router advertises the link prefix as onlink prefix (without that, all traffic goes through the router)

I would love to have ra parameters within the ipv6 parameters, but we use ipv6 to specify the IPv6 address (= string), and it would be too much hassle to deal with one or the other.