sys/gnrc/sock: make use of mbox_size() and mbox_avail()
sys/gnrc/sock: make use of mbox_size() and mbox_avail()

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