gnrc_sock: implicitly set netif if there is only a single one
gnrc_sock: implicitly set netif if there is only a single one

miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT