revert r43582, r49173 and r49177.

open3 arguments uses spawn-like keyword arguments.
Both symbol and integer keys are used.
```
Open3.capture2(*command, :in => IO::NULL, 3 => IO::NULL)
``

This style cannot be supported with keyword splat (**) since Ruby 2.6.
Because Ruby 2.6 prohibits symbol/non-symbol key hash separation.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66352 b2dd03c8-39d4-4d8f-98ff-823fe69b080e