added coap_address_isany() that checks if its argument denotes the ANY address
added coap_address_isany() that checks if its argument denotes the ANY address

authmillenon/libcoap,nmacs/libcoap,gebart/libcoap,nmacs/libcoap,authmillenon/libcoap,authmillenon/libcoap,sgso/libcoap,gebart/libcoap,fredericgermain/libcoap,nmacs/libcoap,gebart/libcoap,thecodemaiden/libcoap-fork,RIOT-OS/libcoap,RIOT-OS/libcoap,sgso/libcoap,thecodemaiden/libcoap-fork,fredericgermain/libcoap,fredericgermain/libcoap,sgso/libcoap