git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@27663 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
returns "::ffff:a.b.c.d" for IPv4-mapped IPv6 address.
[ruby-dev:41215] patch by Tomoyuki Chikanaga.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@27663 b2dd03c8-39d4-4d8f-98ff-823fe69b080e