Revert "Merge pull request #615 from pinwing/dhcp6c"

Asus has changed the dhcp6c_state_main function to not (re)start radvd if nothing has changed.
As a side-effect, when dhcp6c is stopped and this fuction is called, it will not cause radvd to be started, meaning the changes to #615 are not necessary anymore.

(#616 is also unnecessary since radvd is never restarted with the recent changes.)