LWIP_SOCKET_OFFSET macro

LWIP_SOCKET_OFFSET==n: Increases the file descriptor number created by LwIP with n. This can be useful when there are multiple APIs which create file descriptors. When they all start with a different offset and you won't make them overlap you can re implement read/write/close/ioctl/fnctl to send the requested action to the right library (sharing select will need more work though).