sample: use IO::NULL

* sample/cgi-session-pstore.rb: use IO::NULL instead of hard coded
  device name for portability.

* sample/drb/ring_place.rb: ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e