unix, windows: do not set SO_REUSEADDR by default on udp

Add UV_UDP_REUSEADDR flag instead, which can be passed to uv_udp_bind.
If the udp handle is unbound when uv_udp_set_memberhsip or
uv_udp_set_multicast_interface is called, the handle will be bound with
UV_UDP_REUSEADDR set.
unix: add UV_HANDLE_IPV6 flag to tcp and udp handles
bsd: support IPv6 qualified link-local addresses