network: handle captive portal with multiple routers

network: handle captive portal with multiple routers

Before this patch, if a network has multiple routers and one of them
provides a captive portal, then the portal was overwritten or cleared
when another RA from another router is received.

This makes captive portals managed in the similar way as DNS servers or
DNS domains. So now captive portal can safely handled even if a network
has multiple routers.