windows: extend uv_udp_set_membership for IPv6 multicast.

Added uv_udp_set_membership6.
Modified uv_udp_set_membership to call uv_udp_set_membership6
  when IPv6 is used.
Added ifIndex to struct uv_interface_address_s for IPv6 multicasting.

Signed-off-by: snoj <josh@snoj.us>