filehandler.rb: revert r20152

* lib/webrick/httpservlet/filehandler.rb (set_dir_list): revert r20152
  partially and fix misuse of bytesize and regexp repetition operator.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e