radvd service: fix due to systemd upgrade

After the systemd 237 upgrade, radvd wouldn't start anymore because the
PID file cannot be written. It seems that directories in /run has to be
explicitely defined as RuntimeDirectory now. The PID file isn't needed
due to systemd, though, so it was removed along with forking and loggia
via syslog.

This fixes the ipv6 NixOS test.