command-processor.rb: return executable file only

* lib/shell/command-processor.rb (Shell::CommandProcessor#find_system_command):
  return executable file only, should ignore directories and
  unexecutable files.  [ruby-core:57235] [Bug #8918]



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42962 b2dd03c8-39d4-4d8f-98ff-823fe69b080e