child_process: add string shortcut for fork stdio

Add string shortcut option for stdio parameter.

Fixes: https://github.com/nodejs/node/issues/10793
PR-URL: https://github.com/nodejs/node/pull/10866


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Brian White <mscdex@mscdex.net>