[tproxy] errno 92

Hi, just a note, please add an explanation to the README of the upcoming tproxy version, thanks. When iptable_tproxy is not loaded, the setsockopt() calls might return errno 92 (ENOPROTOOPT -- Protocol not available). A quite misleading value, especially since tproxy's setsockopt function returns -ENOPROTOOPT if an invalid itp.op was specified. But given that -ENOPROTOOPT also shows up when tproxy is not loaded, I assume that the general linux networking also returns -ENOPROTOOPT, which makes it almost indistinguishable where the problem is. Jan Engelhardt --