Rename our definition of memmem to dht_memmem.
Rename our definition of memmem to dht_memmem.

This is needed in order to compile on systems that actually have memmem
but don't define HAVE_MEMMEM.

tempbottle/dht-1,loadletter/dht256,mikedld/dht,jech/dht,Erkan-Yilmaz/dht,Librevault/dht,gabrielfalcao/dht,cfpp2p/dht,raspopov/dht