Fix tinc event loop reentrancy from timeout handlers.

Commit 611217c96e introduced a regression
because it accidentally reordered the timeout handler calls and the
fdset setup code. This means that any io_add(), io_del() or io_set()
calls in timeout handlers would be ignored in the current event loop
iteration, resulting in erratic behavior.

The most visible symptom is when a metaconnection timeout occurs and the
connection is closed; the timeout handler closes the socket but it still
ends up in the select() call, typically resulting in the following
crash:

    Error while waiting for input: Bad file descriptor

src/event.c

@@ -256,10 +256,10 @@ bool event_loop(void) {
 	fd_set writable;
 
 	while(running) {
-		memcpy(&readable, &readfds, sizeof readable);
-		memcpy(&writable, &writefds, sizeof writable);
 		struct timeval diff;
 		struct timeval *tv = get_time_remaining(&diff);
+		memcpy(&readable, &readfds, sizeof readable);
+		memcpy(&writable, &writefds, sizeof writable);
 
 		int fds = 0;