std::process::Command::{stdin,stdout,stderr} should specify default behavior

[frewsxcv]

• https://doc.rust-lang.org/std/process/struct.Command.html#method.stdin
• https://doc.rust-lang.org/std/process/struct.Command.html#method.stdout
• https://doc.rust-lang.org/std/process/struct.Command.html#method.stderr

These should specify what the default behavior is if they aren't explicitly called.

[retep998]

Note that the default behavior depends on whether the child process is using the console subsystem or windows subsystem. Console subsystem apps will inherit your stdout/stdin/stderr while windows subsystem apps will start with no stdout/stdin/stderr unless you explicitly assign them.

[steveklabnik]

I've folded this into #29370, let's close this one