Put proxy support back in

[flashmob]

Even though proxying via Nginx is not necessary these days, proxy support may still be useful for load balancing purposes.

The proxy server uses the XCLIENT command to tell the server behind the proxy what the source IP address is.

Requirements:

Support the XCLIENT command (with an option to enable it in config).

case strings.Index(cmd, "XCLIENT") == 0:
				// Nginx sends this
				// XCLIENT ADDR=212.96.64.216 NAME=[UNAVAILABLE]
				client.Address = input[13:]
				client.Address = client.Address[0:strings.Index(client.Address, " ")]
				fmt.Println("client address:[" + client.Address + "]")
				responseAdd(client, "250 OK")