Fix tinc event loop reentrancy from timeout handlers.

Commit 611217c96ec684799882cf330f40a0936131b6b5 introduced a regression
because it accidently 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