struct.c: rb_struct_define_under

* struct.c (rb_struct_define_under): new function to define Struct
  under the given namespace, not under Struct.  [Feature #8264]
* ext/etc/etc.c: use rb_struct_define_under.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42348 b2dd03c8-39d4-4d8f-98ff-823fe69b080e