sys/ps: use getters for thread_t fields
sys/ps: use getters for thread_t fields

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