Add the router preference defines to the icmp6.h header and remove the defines that were part of rtadvd's code.
Add the router preference defines to the icmp6.h header and remove the
defines that were part of rtadvd's code.

ok bluhm@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars