This becomes necesary if sockets become non-blocking by
default <https://bugs.ruby-lang.org/issues/14968>; but it's
always been possible to make sockets non-blocking anyways.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65619 b2dd03c8-39d4-4d8f-98ff-823fe69b080e