ip(8) accepts -family ipv6 (-6) option at the toplevel. It is
straightforward to support the existing option for modifying listener
on IPv6 addresses.

Maintain the backward compatibility by leaving ip fou -6 flag
implemented, while it's removed from the usage message.

Signed-off-by: Sorah Fukumori <her@sorah.jp>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>