timeout.rb: current_target

* lib/timeout.rb (Timeout::ExitException#target): rename attribute.

* lib/timeout.rb (Timeout::ExitException.current_target): extract a
  method to achieve the target.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44520 b2dd03c8-39d4-4d8f-98ff-823fe69b080e