* ext/io/nonblock/nonblock.c (io_nonblock_set): return whether
  nonblock flag was changed.

* ext/io/nonblock/nonblock.c (rb_io_nonblock_block): nothing to
  restore but just yield unless nonblock flag is changed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50531 b2dd03c8-39d4-4d8f-98ff-823fe69b080e