ECN support

[the8472]

Not really a feature request, just an idea that came up when discussing custom UDP transports:

At least on linux (edit: also on windows ) there is the IP_RECVTOS socket option that allow reading TOS and thus ECN flags for received packets, which could be used to implement ECN in user space. That way routers won't be forced to drop uTP packets more aggressively than they do for TCP connections.