We no longer use it this function, but extensions do, and
we need to ensure it continues to work for them.

* thread.c (rb_thread_fd_close): schedule other threads in loop
* ext/-test-/thread_fd_close/thread_fd_close.c: new file
* ext/-test-/thread_fd_close/depend: ditto
* ext/-test-/thread_fd_close/extconf.rb: ditto
* test/-ext-/thread_fd_close/test_thread_fd_close.rb: new test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59030 b2dd03c8-39d4-4d8f-98ff-823fe69b080e