unix, windows: extend uv_udp_set_membership for IPv6 multicast.

Add uv_udp_set_membership6.
Modify uv_udp_set_membership to call uv_udp_set_membership6
  when IPv6 is used.
Add if_index to struct uv_interface_address_s for IPv6 multicasting.
Add UV_HANDLE_IPV6 to unix/interna.h

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