Introduce RTM_CHGADDRATTR to inform userland on the route socket when

Introduce RTM_CHGADDRATTR to inform userland on the route socket when
an attribute of an address is changed.
For now it's used when IPv6 duplicate address detection finishes.

With this slaacd(8) can find out if a configured address is not
duplicated without the need to poll.

OK phessler, benno, claudio