Log value of 'writable' in "could not write to passthru socket" log msg.

May help determine if the socket is disconnected or just not writable.

src/sbbs3/main.cpp

@@ -2053,8 +2053,8 @@ void input_thread(void *arg)
 			if(socket_check(sbbs->passthru_socket, NULL, &writable, 1000) && writable)
 				sendsocket(sbbs->passthru_socket, (char*)wrbuf, wr);
 			else
-				lprintf(LOG_WARNING, "Node %d could not write to passthru socket"
-					, sbbs->cfg.node_num);
+				lprintf(LOG_WARNING, "Node %d could not write to passthru socket (writable=%d)"
+					, sbbs->cfg.node_num, (int)writable);
 			continue;
 		}