Mark all structs `non_exhaustive` for 1.0

[tgross35]

There are a handful of structs that users should be able to access but possibly not construct in case fields change in the future. Currently we often do this by having a single private field, but maybe #[non_exhaustive] would be better suited here.

[tgross35]

We talked about this at rustweek; we would like to use #[non_exhaustive] by default, with some very minor exceptions (e.g. iovec)

[tgross35]

For how to implement this: the s! and s_no_extra_traits! macros should be updated to emit #[non_exhaustive] by default. Then the macro should accept something like #[not_non_exhaustive] to opt out.

[tgross35]

Also, as discussed at #t-libs > Questions and polls about `libc` 1.0 @ 💬, we should be able to derive Default.