Allows setting the advertised route lifetime, and also to set
it to 0 for advertising only the prefix.
From RFC4861:
A router might want to send Router Advertisements without advertising
itself as a default router. For instance, a router might advertise
prefixes for stateless address autoconfiguration while not wishing to
forward packets. Such a router sets the Router Lifetime field in
outgoing advertisements to zero.
bedcc5fc04