child_process: rename _validateStdtio to getValidStdio

The name indicated only validation while it did much more and it
returned a different value to the callee function. The underscore
was also not necessary as the function is internal one way or the
other.

PR-URL: https://github.com/nodejs/node/pull/26809


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>