Use custom methods for values whose type is different on platforms

Use Socket::Option#ip_multicast_loop and Socket::Option#ip_multicast_ttl
instead of Socket::Option#int because NetBSD's size is byte though
others' is int.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41010 b2dd03c8-39d4-4d8f-98ff-823fe69b080e