#29 patch for avoiding memory leaks when add devices

[yongquan kang]

each time a device been added, UpnpInit() is called, on exit, UpnpFinish() is called, but the memories allocated by ThreadPoolInit() may lost because there's no code to call ThreadPoolShutdown() to release the memories. And inet_ntoa() is not thread safe, so in my patch, I substitute inet_ntoa() with inet_ntop(). Your comments are welcome, thank you!

[yongquan kang]

patch for libupnp_1.6.0