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/connection.h

@@ -90,8 +90,7 @@ typedef struct connection_t {
 
 	struct buffer_t inbuf;
 	struct buffer_t outbuf;
-	struct event inevent;           /* input event on this metadata connection */
-	struct event outevent;          /* output event on this metadata connection */
+	io_t io;                        /* input/output event on this metadata connection */
 	int tcplen;                     /* length of incoming TCPpacket */
 	int allow_request;              /* defined if there's only one request possible */