implement COAP_MALLOC_TYPE on lwip
implement COAP_MALLOC_TYPE on lwip

the approach fits very well with the lwippools, and only requires
minimal changes there.

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