child_process: improve ChildProcess validation

This commit improves input validation for the ChildProcess
internals. It became officially supported API a while back, but
never had any validation.

Refs: https://github.com/nodejs/node/issues/12177
PR-URL: https://github.com/nodejs/node/pull/12348


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>