bpo-31922: Do not connect UDP sockets when broadcast is allowed (GH-423)

bpo-31922: Do not connect UDP sockets when broadcast is allowed (GH-423)

*Moved from python/asyncio#493.*

This PR fixes issue python/asyncio#480, as explained in [this comment](https://github.com/python/asyncio/issues/480#issuecomment-278703828).

The `_SelectorDatagramTransport.sendto` method has to be modified ~~so `_sock.sendto` is used in all cases (because it is tricky to reliably tell if the socket is connected or not). Could that be an issue for connected sockets?~~ *EDIT* ... so `_sock.send` is used only if `_sock` is connected.

It also protects `socket.getsockname` against `OSError` in `_SelectorTransport`. This might happen on Windows if the socket is not connected (e.g. for UDP broadcasting).

https://bugs.python.org/issue31922