Lint against #[cfg(linux)] → #[cfg(unix)]

[oberien]

There have been several instances where I wrote #[cfg(linux)] to feature-gate something to linux. This is wrong and will never be evaluated to true. Instead #[cfg(unix)] must be used.

[JamesHinshelwood]

Another possibility could be that #[cfg(target_os="linux")] was intended. I guess we could suggest both?

[euclio]

I also think it would be useful to lint #[cfg(macos)] -> #[cfg(target_os="macos")].