Support HAProxy's PROXY Protocol

[elyscape]

The HAProxy team devised a TCP-level analogue to HTTP's X-Forwarded-For header called PROXY Protocol. It works by including a header to TCP connections with information about the true source of the connection. As the name implies, this is useful when running a TCP service behind a load balancer or reverse proxy. It would be nice if Csocket supported this, providing a method that would cause a listening socket to expect the PROXY Protocol header and use the information provided there for things like Csock::GetRemoteIP().