sd-radv: allow to send multiple routes or prefix64 that have intersection with others

sd-radv: allow to send multiple routes or prefix64 that have intersection with others

I cannot find any RFC that states we should not send multiple route
prefix or pref64 options that have intersection with others.
Moreover, each route prefix option has preference field, thus, user may
want to send e.g. a prefix with the normal preference, and another sub
prefix with the high preference. Previously, such configuration was
prohibited. Let's allow that now.