HTTP downloader port

src/netcode/client_connection.c

src/netcode/client_connection.h

src/netcode/d_clisrv.c

src/netcode/d_clisrv.h

src/netcode/d_netcmd.c

src/netcode/d_netfil.c

src/netcode/d_netfil.h

src/netcode/protocol.h

src/netcode/server_connection.c