LWIP_FIONREAD_LINUXMODE macro

LWIP_FIONREAD_LINUXMODE==0 (default): ioctl/FIONREAD returns the amount of pending data in the network buffer. This is the way windows does it. It's the default for lwIP since it is smaller. LWIP_FIONREAD_LINUXMODE==1: ioctl/FIONREAD returns the size of the next pending datagram in bytes. This is the way linux does it. This code is only here for compatibility.