Remove the TAP-Win32 reader thread.

tinc is using a separate thread to read from the TAP device on Windows.
The rationale was that the notification mechanism for packets arriving
on the virtual network device is based on Win32 events, and the event
loop did not support listening to these events.

Thanks to recent improvements, this event loop limitation has been
lifted. Therefore we can get rid of the separate thread and simply add
the Win32 "incoming packet" event to the event loop, just like a socket.
The result is cleaner code that's easier to reason about.