discard from socket using recv MSG_TRUNC on Linux TCP SOCK_STREAM socket
Currently, lighttpd supports only TCP SOCK_STREAM. If UDP SOCK_DGRAM
were to be supported in the future, then socket type will need to be
stored so that MSG_TRUNC is used appropriately for the desired effect.
To find out socket type on arbitrary socket fd:
getsockopt(..., SOL_SOCKET, SO_TYPE, ...)
but better to store it with each listening socket.