Shenanigans with `Cmd::stdin` in `0.3.0-pre.2`

[cyqsimon]

Cmd::run's documentation claims:

command's stdin is set to null

But that's not true when Cmd::stdin has been used.

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Also confusing is the fact that Cmd::run_echo has a completely different behaviour, namely that it doesn't respect Cmd::stdin at all, despite the naming suggesting otherwise.