Some IOCP bufferevent tweaks.

Some IOCP bufferevent tweaks.

- Increment reference count of bufferevents before initiating overlapped
  operations to prevent the destructor from being called while operations
  are pending. The only portable way of canceling overlapped ops is to
  close the socket.

- Translate error codes to WSA* codes.

- Better handling of errors.

- Add an interface to add and del "virtual" events. Because IOCP
  bufferevents don't register any events with the base, the event loop
  has no way of knowing they exist. This causes the loop to terminate
  prematurely. event_base_{add,del}_virtual increment/decrement base's
  event count so the loop runs while there are any enabled IOCP
  bufferevents.