currently, it's possible to enable SLAAC (accept_rtadv) but disable
automatic default router configuration using the nd6 flag no_radr.
add a new flag, no_prefixaddr, which does the inverse: automatically
configure the default router, but don't configure any IP addresses, even
if an autonomous on-link prefix is advertised.
this decision is made late enough that we still configure the prefix
itself and handle updates to any autoconf addresses that may already
exist on the interface, but we don't configure a new address when we
usually would.
this flag is disabled by default, so existing behaviour is unchanged.
this conforms to RFC 4862 § 5.5:
Creation of global addresses as described in this section SHOULD be locally configurable. However, the processing described below MUST be enabled by default.