Support inheriting stdin

[RalfJung]

Sometimes one really wants stdin to be inherited from the parent process. Currently, that does not seem to be possible with xshell, so I have to use std::process instead, thus losing all the other nice features of xshell. Would be nice if xshell could support inheriting stdin :)

[Tudyx]

I think that's what the (not released yet) run_interactive method do