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merge revision(s) 57531,57532,57533,57537: [Backport #13191]

    pty/shl.rb: update [ci skip]

    * sample/pty/shl.rb: stop writer loop when the child exited.
      PTY::ChildExited no longer raises asynchronously since r20298.
      [ruby-dev:49974] [Bug #13191]
    pty/shl.rb: update [ci skip]

    * sample/pty/shl.rb: use io/console instead of stty.
      [ruby-dev:49974] [Bug #13191]
    pty/shl.rb: update [ci skip]

    * sample/pty/shl.rb: do not manage array length separately.
      [ruby-dev:49974] [Bug #13191]
    pty/shl.rb: update [ci skip]

    * sample/pty/shl.rb: leap exited child process.
      [ruby-dev:49974] [Bug #13191]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@58620 b2dd03c8-39d4-4d8f-98ff-823fe69b080e