* tool/downloader.rb: make Downloader class to general download utility.
  It can be used without config.guess and configu.sub.
* tool/get-config_files: ditto.
* tool/make-snapshot: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46535 b2dd03c8-39d4-4d8f-98ff-823fe69b080e