sliptty/start_network.sh: don't default to DHCPv6
sliptty/start_network.sh: don't default to DHCPv6

DHCPv6 is enabled by the `-d` option.
It should not be enabled in absence of this option.

ant9000/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,basilfx/RIOT,yogo1212/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,basilfx/RIOT,jasonatran/RIOT,kaspar030/RIOT,OTAkeys/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,basilfx/RIOT,miri64/RIOT,yogo1212/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,basilfx/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,yogo1212/RIOT,authmillenon/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,yogo1212/RIOT,miri64/RIOT,kaspar030/RIOT,jasonatran/RIOT,basilfx/RIOT,kaspar030/RIOT,kYc0o/RIOT,yogo1212/RIOT