coap: add Proxy-* options
coap: add Proxy-* options

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