Move contents of win_seat_notify_remote_exit into a callback.

This matches the way it's done in the GTK backend: the only thing that
happens inside seat_notify_remote_exit is that we schedule a toplevel
callback, and all the real work happens later on when that callback is
called.

The particular situation where this makes a difference is if you
perform a user abort during proxy authentication (e.g. hit ^D at a
proxy password prompt), so that the main SSH backend gets
PLUGCLOSE_USER_ABORT and calls ssh_user_close. That doesn't
immediately close the socket; it just sets ssh->pending_close,
schedules a run of ssh_bpp_output_raw_data_callback, and returns.

So if seat_notify_remote_exit calls back _synchronously_ to
ssh_return_exitcode, it will see that the socket is still open and
return -1. But if it schedules a callback and waits, then the callback
to ssh_bpp_output_raw_data_callback will happen first, which will
close the socket, and then the exit processing will get the right
answer.

So the user-visible effect is that if you ^D a proxy auth prompt, GTK
PuTTY will close the whole window (assuming you didn't set close-on-
exit to 'never'), whereas Windows PuTTY will leave the window open,
and not even know that the connection is now shut (in that it'll still
ask 'Are you sure you want to close this session?' if you try to close
it by hand).

With this fix, Windows PuTTY behaves the same as GTK in this respect.

windows/window.c

@@ -2106,7 +2106,7 @@ static bool is_alt_pressed(void)
 
 static bool resizing;
 
-static void win_seat_notify_remote_exit(Seat *seat)
+static void exit_callback(void *vctx)
 {
     int exitcode, close_on_exit;
 
@@ -2133,6 +2133,11 @@ static void win_seat_notify_remote_exit(Seat *seat)
     }
 }
 
+static void win_seat_notify_remote_exit(Seat *seat)
+{
+    queue_toplevel_callback(exit_callback, NULL);
+}
+
 void timer_change_notify(unsigned long next)
 {
     unsigned long now = GETTICKCOUNT();