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.