Always set autoclose=false for IO.for_fd fds

* I believe this should be default behavior, see [Feature #2250].
* Now make test-spec MSPECOPT='-R100 spec/ruby/library/socket' works fine.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64454 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/socket/unixsocket/recvfrom_spec.rb

@@ -77,6 +77,7 @@ with_feature :unix_socket do
         @server_raw.bind(Socket.sockaddr_un(@path2))
 
         @socket = UNIXSocket.for_fd(@server_raw.fileno)
+        @socket.autoclose = false
       end
 
       after do