There's a chance that between spawning of the `std Child`, and wrapping
it into a tokio impl an error can occurs, which would leave the child
alive, with no way to access it from the user side and not being part
of the tokio runtime.

This commit fixes that issue by wrapping the `std Child` right after
spawning with a struct that implements Drop, where the child would
be killed if it's dropped.

Fixes: tokio-rs#6797