Rewrite src/apps/gateway.c to work with 64bit compilation + redesign for better maintenance.
Some small other modifications in Makefile and src/apps/client.c for 64bit compatibility.
1) BUGFIX: Fixed the return value of IP_send() and IP_recv() so that they does not include the header size anymore.
2) ADD: Harmonization of IP_send() and IP_recv() so that they both use mic_tcp_pdu data instead of ip_payload data.
Moved therefore the ip_payload type declaration in mictcp_core.h.
3) ADD: Add the source address to the arguments of process_received_PDU() in order to make the multiplexing implementation possible.
4) ADD: Generate a stub addr in IP_recv()
