Termios needs a safe constructor

[asomers]

The only constructor method for Termios is the unsafe and undocumented default_uninit. All of the examples use that function. But not only is it unsafe, it also uses std::mem::uninitialized, which will be deprecated in Rust 1.38.0. This will soon be the last remaining use of std::mem::uninitialized.

[asomers]

ping @Susurrus would you have some time to try fixing this?

[Susurrus]

Is there anything to fix? #1158 got rid of mem::uninitialized(), and zeroing it seems fine. If anything switching it from doc(hidden) to pub(crate) seems more useful (same with the below update_wrapper() function.

[asomers]

Is all zeros actually a valid state for a libc::termios? If so, then zeroing it is ok. But if not then we should just delete that function and force people to use proper constructors like tcgetattr. Also, we should remove default_uninit from the API docs.

[Susurrus]

This is effectively a private function because of the doc(hidden) (which was the way to do this before pub(crate)). That's why I suggested changing it to pub(crate) and being done (however this is technically a breaking change). I don't use it in serialport, which would likely be one of the only users, so I imagine it won't impact users much.

[asomers]

Yeah, I agree with making it pub(crate). But even though it's currently #[doc(hidden)], it's still used in the API docs (in hidden form), so you'll have to replace it with something or the doc tests will break.