Use the whole allocated buff for network transfers

The Ethernet buffer is defined in defines.h as 850 octets, but in server.ino a hard-coded value of 512 octets is used for the file transfers. This patch increases the buffer utilization in file transfers. It also corrects the frame length in the enc26j60 driver to allow the full 1500 octet payload for the ATmega1284. Transfers are significantly faster as a result.