unix, win: 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.

Google groups discussion...not much there mainly easy access for myself. https://groups.google.com/forum/?fromgroups=#!topic/libuv/TxZmEnH2wP4