router: implement RFC 7084 errata 7699

[alinnastac]

RFC 7084 L-3 requires router advertisement daemon to send RIO for every prefix delegation that allocates a prefix on the interface. There is one special case though where PIO advertise the on-link prefix route that conflicts with the RIO prefix.

Example:

• CE-Router receives IA-PD 2001:db8:1234::/64
• this prefix delegation gets assigned to the lan interface
• odhcpd advertise on-link PIO with prefix 2001:db8:1234::/64

If original RFC 7084 L-3 would be followed, RA will also contain a RIO with prefix 2001:db8:1234::/64 which will require lan hosts to add the routes 2001:db8:1234::/64 dev br-lan # the on-link prefix route 2001:db8:1234::/64 dev br-lan nexthop fe80::1 # RIO route