Don't use undefined socket.IPPROTO_IPV6 on Windows
Alexandre reports this crash on Windows 7 with Python 2.7.3:
Traceback (most recent call last):
File "flashproxy-client", line 1003, in <module>
local_listen.append(listen_socket(addr))
File "flashproxy-client", line 464, in listen_socket
s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 1)
AttributeError: 'module' object has no attribute 'IPPROTO_IPV6'
He points to http://bugs.python.org/issue6926. We can safely skip this statement on Windows because IPV6_V6ONLY
is the default for Windows sockets.