CmsgSpace::new unsafely creates uninitialized, arbitrary types

[goffrie]

Even though the uninitialized field is private, it still gets dropped. For example nix::sys::socket::CmsgSpace::<String>::new(); easily causes a segfault.

The field should probably be ManuallyDrop<T>, or there should be a T: Copy bound.

[asomers]

Good catch. Unfortunately, we can't require Copy. If we did, our own tests wouldn't compile! That's because libc structs aren't Copy. Would you like to submit a PR that implements the ManuallyDrop solution?

[asomers]

Nevermind. I'm going to eliminate CmsgSpace instead.