webrick/httpserver.rb: Stop handling requests on shutdown

* lib/webrick/httpserver.rb (WEBrick::HTTPServer#run): stop
  handling requests on shutdown, even if the socket is readable
  and IO.select() returns true.  [Fixes GH-607]
* lib/webrick/server.rb (WEBrick::GenericServer#start): IO.select()
  raises ENOTSOCK on shutdown on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45872 b2dd03c8-39d4-4d8f-98ff-823fe69b080e