Drop libevent and use our own event handling again.

There are several reasons for this:

- MacOS/X doesn't support polling the tap device using kqueue, requiring a
  workaround to fall back to select().
- On Windows only sockets are properly handled, therefore tinc uses a second
  thread that does a blocking ReadFile() on the TAP-Win32/64 device. However,
  this does not mix well with libevent.
- Libevent, event just the core, is quite large, and although it is easy to get
  and install on many platforms, it can be a burden.
- Libev is more lightweight and seems technically superior, but it doesn't
  abstract away all the platform differences (for example, async events are not
  supported on Windows).

src/tincd.c

@@ -450,18 +450,6 @@ int main2(int argc, char **argv) {
 	}
 #endif
 
-#ifdef HAVE_DARWIN
-	if(!getenv("EVENT_KQUEUE"))
-		setenv("EVENT_NOKQUEUE", "1", 0);
-	if(!getenv("EVENT_POLL"))
-		setenv("EVENT_NOPOLL", "1", 0);
-#endif
-
-	if(!event_init()) {
-		logger(DEBUG_ALWAYS, LOG_ERR, "Error initializing libevent!");
-		return 1;
-	}
-
 	/* Setup sockets and open device. */
 
 	if(!setup_network())