ifdef SIOCGIFHARDMTU (for compile with openbsd 5.2)
Put rtadv back into ctl framework, but only for config file editing.
Point interface rtadvd to rtadv config file, which can handle multiple
interfaces.

This way you can do 'rtadv edit' and setup all the awesome termcap-like
entries. Then, int xyz0 and rtadvd, int xyz1, rtadvd, etc...
add ip6 accept-rtadv sysctl