Add support for proxying through an external command.

Proxy type "exec" can be used to have an external script or binary set
up an outgoing connection. Standard input and output will be used to
exchange data with the external command. The variables REMOTEADDRESS and
REMOTEPORT are set to the intended destination address and port.

src/protocol_auth.c

@@ -117,6 +117,8 @@ static bool send_proxyrequest(connection_t *c) {
 		case PROXY_SOCKS4A:
 			logger(LOG_ERR, "Proxy type not implemented yet");
 			return false;
+		case PROXY_EXEC:
+			return true;
 		default:
 			logger(LOG_ERR, "Unknown proxy type");
 			return false;